Adds multi-theme support using the new THEMES setting.
You can specify all the themes that you will be using in python
dicionary form. You can then inherit from the themes specified
in THEMES using the corresponding key in the dictionary.
The locale is a global state, and it was not properly reset to
whatever it was before the unitttest possibly changed it.
This is now fixed.
Not restoring the locale led to weird issues: depending on
the order chosen by "python -m unittest discover" to run
the unit tests, some tests would apparently randomly fail
due to the locale not being what was expected.
For example, test_period_in_timeperiod_archive would
call mock('posts/1970/ 1月/index.html',...) instead of
expected mock('posts/1970/Jan/index.html',...) and fail.
This dictionary is accessed by plugins (like `summary`) which add new
settings, so it should be public (i.e. no prefixed underscore).
The changed name length would have led to a re-indenting of the
default contents anyway, so I shifted them all to four spaces.
