Internationalization

Basic skeleton for internationalization. Added:
 * Django i18n support
 *  an `extract_database_strings` command
 * Docs (!) about how things work.

This depends on merging #5 first.
Sure I can switch that back to a model attribute. I was just experimenting but maybe it makes more sense to just have an attribute. FeinCMS makes heavy use of fein_* attributes too.