Make mailpile auto-recognize the locale path

Currently I can run MP only from the Mailpile clone directory, because it doesn't properly recognize the locale directory. *gettext* searches in *[current CWD]/locale* which obviously can't work.

This Pull Request:
* Installs the entire *locale* directory in setup.py, just like the *static* directory
* Uses the locale directory in *[path to config.py]/../locale* in any case.

So, if you're running an installed version of Mailpile, it will work from everywhere (at least regarding that aspect).
If you're running mailpile from the clone directly, the behaviour will be the same as before.

Tested on Ubuntu 13.04 x64.