Danbooru2 theme, tag categories extension, tag list update/rework

Hi Shish,

This pull request basically adds tag categories, similar to Danbooru's `show:Lucky_Star` , `character:Someone_Nanoka` and all that. It does this by hooking into the tag_list extension, to colour the tags with categories. Along with this, it also provides an interface to change tag categories.

It also adds a Danbooru2 theme, which modifies the Danbooru theme to look and act more closely to the current Danbooru site/software. New theme and name changed to Danbooru2 at the request/offer of Shish in IRC. 

Screenshots:
![Display on the post listing](https://f.cloud.github.com/assets/251281/662062/24b22790-d74c-11e2-9d05-14afcf8c39ce.png)
![Default display on post pages](https://f.cloud.github.com/assets/251281/662063/2738433c-d74c-11e2-820a-8193ee3238c1.png)
![Category modification page, tags/catetgories](https://f.cloud.github.com/assets/251281/662064/298045b8-d74c-11e2-8186-1bbfba4c9ed9.png)