Support Multiple themes

[tsiemens]

I've seen other apps with this, but I don't know the demand.
Fairly trivial to implement, just need to add a base activity (or multiple, since need a separate one for preference activity) which calls setTheme before super.onCreate

The more tedious part is to make or add dark versions of all our icons, and change all icon references and colour values to use a ?attr value, as described here http://stackoverflow.com/questions/13735675/change-drawable-based-on-theme

Obviously, would also need to add an item to the preferences page. Right now, would probably limit to holo light, holo dark, light with dark actionbar. later could maybe add a parchment theme, like our competitors seems to use.