Merge pull request #1 from jeffcarp/master

Add .extras attribute to Model
Update documentation.
Bump version to 0.2.0
Update documentation.
Update documentation.
Refactor relations API.
Add aliases for relation methods.
Default attributes to null instead of undefined.
Don't validate null attributes (use 'required' validator).
Bump version to 0.3.1
Do not mark unchanged attributes as dirty on initialization.
Add support for generators. Closes #3.

If the visionmedia/co module is resolvable or `options.thunks` is set,
callback methods are wrapped to return a thunk for generator flow control.
Bump version to 0.4.0
Add relationship to through model.
Add Model.browser() and Model.server()
Return model for Model.browser() or Model.server()
Fix bug with type check
Fix bug with marking default attributes as dirty.
Bump version to 0.5.4
Fix bug with overwriting default attributes.