Refactor library internals

[EvanHahn]

This could be done any number of ways, but I don't like that there's one big test file and one big source file (this echoes @janmarek's comments in #6).

This is my ideal directory structure, but I could see it going other ways:

expect.js
package.json
lib/
├── Assertion.js
└── assertions/
    ├── be.js
    ├── equal.js
    ├── greaterThan.js
    ├── lessThan.js
    └── ...

Thoughts?