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.
Never mind, I forgot that we can't add arbitrary attributes to the inner Meta class without hackery that isn't worth it.  The private model attribute you were using before could still work though. There's some additional refactoring I'd like to do as well; I'll follow up with that in the form of a PR.