Add new _parler_meta attribute, that will replace _translations_model / _translations_field

It's still compatible with the existing code that may have read the
internal _translations_model attribute
Using _parler_meta internally everywhere now.
Allow construction of translated model, as long as related_name differs
Update the internal caching machinery to support accessing multiple translated models.

- Added `related_name=None` parameters to API functions. Uses base translation by default.
- Reworked `_translations_cache` to support multiple models.
- Store the `__init__()` and `create_translation()` kwargs in the proper model.
- Access `ParlerOptions` in `TranslatedFieldDescriptor` to read the correct model.
- Extra: using delete_many() for memcached now.
Added `include_unsaved` parameter to `get_available_languages()` API.
Add get_translation() API

One of my packages (django-fluent-pages) really needs
this to avoid hacking into the `_translations_cache`.
Simplify ParlerOptions code, keep all model registrations as separate ParlerMeta object
Add proxy model tests
Simplify code, renanme parler_meta to meta everywhere.
forms: show all translated fields in the form.
Replace TypeError with ImproperlyConfigured for _validate_master()
Remove the _translations_model backwards compatibility option.
Make sure that adding a second translation table creates only one ParlerOptions object.
minor cleanups
Fix inheritance check, should only close base class, not subclasses
Merge branch 'inheritance'
Bump version to 1.2