new theme: "minimal"

I wanted to simplify the buddypress-docs UI a little bit, so I put these changes into a new theme. I added an admin section "Appearance" and a setting "theme," which now has two options: Default and Minimal. Selecting "minimal" engages the "minimal" theme, which has these features: 

 * No tabs. An extreme change, perhaps, but I figure they're unnecessary. While on a group docs page, it's not necessary to have a link to "all docs" (this is taken care of by the site-wide docs page), nor is it necessary to show the group's name (i.e. Example Group's Docs). Besides, going to "all docs" from a group page is confusing to the user, who expects to remain in the group page. On the "all docs" listing, a user's "created by me" and "edited by me" docs are handled by that user's profile page. 
 * No tables. Tables are replaced with a cleaner, paragraph-style display. This eliminates redundancy with lines where the "created date" and "last modified date" are the same. 

Screenshot of the revamped doc listing view: 
![screen shot 2014-05-27 at 2 11 20 pm](https://cloud.githubusercontent.com/assets/1843676/3095654/632266a6-e5ca-11e3-9a97-81eeb476c4b2.png)