Initial work to add model translations
Hook up forms to use TranslationModelForm
Hookup tabbed translation in admin for Layers, Maps, Documents
Remove use of tabbed inputs and add admin to documents
Use base MediaTranslationModel
Remove extraneous Media class from DocumentTranslationOptions
Add django-modeltranslation to setup.py
Move MediaTranslationAdmin to a more sensible spot and dont hook up ResourceBase for translation directly (only do it with Layer, Map, Document etc)
Merge pull request #1527 from jj0hns0n/model_translation

Model translation