extract eachTranslatedAttribute

The logic of iterating over ___Translation attributes in an object was
inside the TranslateableAttributes#didInsertElement. This extracts it
to a helper that that method calls.
Add TranslateableProperties mixin

The existing TranslateableAttributes mixin takes ___Translation
properties on the view and turns them into attributes on the view's DOM
element.

The TranslateableProperties mixin takes ___Translation properties on any
Em.Object and turns them into attributes on the object itself.
Merge pull request #54 from jamesarosen/translated_properties

Add TranslateableProperties