ModelBuilder: add prototype.defineValueType

Add a shortcut for registering a new value type.

The current implementation registers the type in the singleton registry
`ModelBuilder.schemaTypes`.

The API should allow us to to change the implementation to register
the type in the scope of ModelBuilder instance only.

lib/model-builder.js

@@ -432,6 +432,15 @@ ModelBuilder.prototype.defineProperty = function (model, propertyName, propertyD
   this.models[model].registerProperty(propertyName);
 };
 
+/**
+ * Define a new value type that can be used in model schemas as a property type.
+ * @param {function()} type Type constructor.
+ * @param {string[]=} aliases Optional list of alternative names for this type.
+ */
+ModelBuilder.prototype.defineValueType = function(type, aliases) {
+  ModelBuilder.registerType(type, aliases);
+};
+
 /**
  * Extend existing model with bunch of properties
  *