Django-bingo

* fix: avoid creating duplicate BingoBoards / Games

* fix: mark "my board" on the game page, too.
* new: added a light theme, which overrides the dark theme, which can be
added to the theme switcher, when the dark theme is the default THEME.
* improved: title-text for board thumbnails with creation time. alt-text
with board_id.
* improved: board thumbnails switch between voted / marked on mouseover.
* new: VOTE_START_TIME allows to set a time between GAME_START_TIMES[0]
and GAME_END_TIME, after which voting will be activated. This can be
used to allow board creation before the game starts, but disable early
votes.
* new: added a footer.

* fix: each BingoBoard was missing one of the words, which are not on the board.

* new feature: show boards as thumbnails in the board list.
* changed: removed is_active. to set a Word to inactive, just remove it from all sites.
* changed: added "unique" constraint to field Word.word.
* improved: added some more list-columns and list-filters in the admin interface.
* new feature: implemented theme support
* THEME = "somewhere/myfile.css" is the path for a site-theme
* THEMES = (('name', 'path/to/file.css'), ...) are themes shown in a theme chooser
* new dark theme included. use "bingo/themes/dark.css" as path.
* Note: you now need to add TEMPLATE_CONTEXT_PROCESSORS to your config, see the README.

* fixed: GAME_END_TIME did not stop currently running games

* performance: optimized queries by using select_related().
* performance: implemented a vote cache, which avoids a lot of queries.
* style: use "background-attachment: fixed" for the background image.
* pep8: urls.py is now pep8 compliant.