Cleanup codebase for work on v0.5

The following changes are included as separate, logically independent commits:
__First commit — farewell nani__
* Renaming of `NaniTestCase`, to `HvadTestCase`.
* Removal of nani, except for the `NANI_TABLE_NAME_SEPARATOR` setting and admin template's nani-language-tabs CSS class, which we forgot to deprecate. This makes them deprecated so they can be ridden of at a later time.
* Update of module-level settings so they update themselves on `django.utils.test.signals.setting_changed`. It allows `TestCase.settings()` overrides to work despite the settings being cached at module level. Required for tests to work as the fallback code to handle `NANI_TABLE_NAME_SEPARATOR` deprecation is at module level.

__Second commit — cleanup__
* Removal of conditional code for Django <= 1.2, as it has been unsupported for one year and does not work anymore anyway.
* Removal of dead code. Mostly backports from Django 1.3 features intended for use with older versions (eg: `hvad.compat.date`, `assertNumQueries`, etc.)

__Third commit — documentation__
* Version bump to 0.5. New entry in release notes marking it as development version so pull requests can update the release notes, will be easier than reviewing the whole history later.
* Removal of `changelog.rst`, as it is just a copy of `public/release_notes.rst` and noone keeps them synced.
* Catch up of updates already merged into master, so the release notes are in sync with the current state of the code.