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.
How about we add a `translated_fields` attr to the inner `Meta` class, and then check

```python
if field.name not in getattr(entry._meta, 'translated_fields', []):
```
instead of

```python
if not isinstance(field, LocalizableField):
```