Refactored the `buildAttributes` method and split it into `validateAttributes` and `removeAttributes`.
Added a few mixins
Added new tests for mixins
Added new tests for model attributes related stuff
added another test
more test coverage
didn’t mean to commit this
more asserts
better comments describing validateAttributes
Renamed all references to ObjectID to ObjectId, removed isValidObjectId in favor of isObjectId
Merge pull request #5 from airbrite/peter/attributes

Refactored the `buildAttributes` method and split it into `validateAttributes` and `removeAttributes`.