Add "useClass" option to inject, find, findAll, create

[jmdobry]

Right now useClass defaults to true, but it set at the resource/global level.

Adding the useClass option to these methods gives greater control, for example:

• DS.find - useClass: true - create instance from attrs, inject into store and return instance
• DS.find - useClass: true, cacheResponse: false - create instance from attrs and return instance, do not inject into store
• DS.find - useClass: false - inject raw attrs into store and return raw attrs
• DS.find - useClass: false, cacheResponse: false - return raw attrs, do not inject into store