Added Sphinx documentation!! and made a few code moves.

- Moved `parler.appsettings.add_default_language_settings()` to `parler.utils.conf`
  (old import is still there).
- Moved `parler.admin.BaseInlineFormSet` to `parler.forms`.
Finetune documentation and README
docs: mention creating models manually
Improve sphinx docs, move more README code to docs
Improve quickstart and README
Update README for clarity
docs: mention switch_language() context manager
docs: mention safe_translation_getter() too
apidocs: fix multiple toctrees
Release 1.0 with full documentation and stability mark.
Separated TranslatableModel into TranslatableModel stub and TranslatableMixin
Compile .mo files on `pip install -e` too.
Removed unused import
For consistency: Renamed TranslatableMixin => TranslatableModelMixin
Testing for instance on TranslatableModelMixin rather than TranslatableModel
Merge pull request #23 from jrief/translatable-mixin

Separated TranslatableModel into TranslatableModel stub and Translatable...
Reverting PR #23, since model separation does not work out with PolymorphicModel
Added a short description how to inherit from a PolymorphicModel
--use-mirrors has been deprecated and will be removed in the future.
Merge pull request #24 from jrief/translatable-mixin

Revert PR #23