Model translation

This PR adds support for translating enumerations in the database like TopicCategory, Region, License etc and also allows for storing many fields of Layers, Maps and Documents in various languages. The Django Model Translation App is used and handles everything in tandem with the normal i18n setup. 

All languages can be edited from the admin in a tabbed interface (see screenshot below) and users can edit metadata for Resources in various languages by submitting the metadata form after changing languages in the app (in footer). It is not currently possible to edit multiple languages for metadata in the actual site all at once, but requires submitting the form once per language. This may be able to be improved.

We will need to figure out if it is possible to manage these translations using transifex

![change_topic_category___django_site_admin](https://cloud.githubusercontent.com/assets/506078/3251380/f4f7e5aa-f1b0-11e3-9b63-10e38ffc7fec.png)
![geonode_single_point__example_com](https://cloud.githubusercontent.com/assets/506078/3251381/f503454e-f1b0-11e3-8dcb-236f557df81b.png)