Fixes #18
Fixes #11 - Category capability

Adds capability to add a category to a post, as well as:
- Editing a post to add a category
- Viewing all posts with the same category
-- Paging within posts of the same category!
- Added category to the list posts admin page
- Added ability to click through on a category and see that categories posts
Merge pull request #19 from holtscomm/categories

Category capability