Added grunt watch task for rebuilding
Changed dist dir cleanup to only happen in 'dist' task
Refactored module structure

* Extracted base model logic into `CoreModel` class
* Moved `DS.Store` and `DS.Model` additions into an 'ext' module
* Created 'ember' module for importing Ember directly
Fixed issue with `hasOneFragment` default values
Fixed issue with setting a `hasOneFragment` property to null
Added limited support for fragments in `DS.Model#changedAttributes`
Fixed issue with owner-less fragments on new records, fixes #3
Added support for empty adapter response, fixes #4
Fixed issue with fragment array methods bypassing `replaceContent`