Add: themes manager

Through the files located in media/themes/, it is now possible to specify
the wanted theme in settings.py
Facilities have been added to easily re-use images and common css directions.

So now I guess we should reorganize a bit what's in media. What about something like:
* media/external/* for external tools (bootstrap, jquery, etc)
* media/themes/* for themes storage
* media/img for shared images
* media/ico for favicons
* media/js for shared javscript

Let me know what you think!