Add jest matcher

Adding jest matchers to conveniently use targaryen with jest.

Usage:
```
const { getDatabase, toAllowRead, toAllowUpdate, toAllowWrite, json } = require('targaryen/plugins/jest');

expect.extend({
  toAllowRead,
  toAllowUpdate,
  toAllowWrite,
});

const RULES_PATH = 'database.rules.json';
const rules = json.loadSync(RULES_PATH);
const initialData = {};

const database = getDatabase(rules, initialData);

test('basic', () => {
  expect(database.as(null)).not.toAllowRead('/user');
  expect(database.as(null)).toAllowRead('/public');
  expect(database.as({ uid: '1234'})).toAllowWrite('/user/1234', {
    name: 'Anna',
  });
});

```

or using the generic matcher:
```
const { getDebugDatabase, toBeAllowed, json } = require('targaryen/plugins/jest');

expect.extend({
  toBeAllowed,
});

const RULES_PATH = 'database.rules.json';
const rules = json.loadSync(RULES_PATH);
const initialData = {};

// NOTE: Create a database with debug set to true for detailed errors
const database = getDebugDatabase(rules, initialData);

test('generic', () => {
  expect(database.as(null).read('/user')).not.toBeAllowed();
  expect(database.as({ uid: '1234' }).write('/user/1234', {
    name: 'Anna',
  })).toBeAllowed();
});

```

plugins/jest.js

@@ -0,0 +1,100 @@
+/**
+ * targaryen/plugins/jest - Reference implementation of a jest plugin for
+ * targaryen.
+ *
+ */
+
+'use strict';
+
+const targaryen = require('../');
+
+function toBeAllowed({ info, allowed }) {
+  const pass = allowed === true;
+  const message = pass
+    ? () => `Expected operation to be ${this.utils.EXPECTED_COLOR('denied')} but it was ${this.utils.RECEIVED_COLOR('allowed')}\n\n${info}`
+    : () => `Expected operation to be ${this.utils.EXPECTED_COLOR('allowed')} but it was ${this.utils.RECEIVED_COLOR('denied')}\n\n${info}`;
+
+    return {
+    message,
+    pass,
+  };
+}
+
+function toAllowRead(database, path, options) {
+  const { info, allowed } = database
+    .with({ debug: true })
+    .read(path, options);
+
+  const pass = allowed === true;
+  const message = pass
+    ? () => `Expected ${this.utils.EXPECTED_COLOR('read')} to be ${this.utils.EXPECTED_COLOR('denied')} but it was ${this.utils.RECEIVED_COLOR('allowed')}\n\n${info}`
+    : () => `Expected ${this.utils.EXPECTED_COLOR('read')} to be ${this.utils.EXPECTED_COLOR('allowed')} but it was ${this.utils.RECEIVED_COLOR('denied')}\n\n${info}`;
+
+  return {
+    message,
+    pass,
+  };
+}
+
+function toAllowWrite(database, path, value, options) {
+  const { info, allowed } = database
+    .with({ debug: true })
+    .write(path, value, options);
+
+  const pass = allowed === true;
+  const message = pass
+    ? () => `Expected ${this.utils.EXPECTED_COLOR('write')} to be ${this.utils.EXPECTED_COLOR('denied')} but it was ${this.utils.RECEIVED_COLOR('allowed')}\n\n${info}`
+    : () => `Expected ${this.utils.EXPECTED_COLOR('write')} to be ${this.utils.EXPECTED_COLOR('allowed')} but it was ${this.utils.RECEIVED_COLOR('denied')}\n\n${info}`;
+
+  return {
+    message,
+    pass,
+  };
+}
+
+function toAllowUpdate(database, path, patch, options) {
+  const { info, allowed } = database
+    .with({ debug: true })
+    .update(path, patch, options);
+
+  const pass = allowed === true;
+  const message = pass
+    ? () => `Expected ${this.utils.EXPECTED_COLOR('update')} to be ${this.utils.EXPECTED_COLOR('denied')} but it was ${this.utils.RECEIVED_COLOR('allowed')}\n\n${info}`
+    : () => `Expected ${this.utils.EXPECTED_COLOR('update')} to be ${this.utils.EXPECTED_COLOR('allowed')} but it was ${this.utils.RECEIVED_COLOR('denied')}\n\n${info}`;
+
+  return {
+    message,
+    pass,
+  };
+}
+
+/**
+ * Simple wrapper for `targaryen.database()` for conveniently creating a
+ * database for a jest test.
+ */
+function getDatabase(...args) {
+  return targaryen.database(...args);
+}
+
+/**
+ * Simple wrapper for `targaryen.database()` that also enables debug mode for
+ * detailed error messages.
+ */
+function getDebugDatabase(...args) {
+  return targaryen.database(...args).with({ debug: true });
+}
+
+const jestTargaryen = {
+  toBeAllowed,
+  toAllowRead,
+  toAllowWrite,
+  toAllowUpdate,
+
+  // NOTE: Exported for convenience only
+  getDatabase,
+  getDebugDatabase,
+  json: require('firebase-json'),
+  users: targaryen.util.users,
+};
+
+module.exports = jestTargaryen;