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

@gholtslander !

At this time, it is only possible to add one category to a post (and have it work, anyway). I am going to add an enhancement for multiple categories, but it doesn't feel necessary at this time, since the main driver behind this functionality was to have "multiple 'blogs' within one blog" as it were.

Dad, if you want you can checkout this branch and pull it down locally and try it out. Please log any Issues you find with it! If you have time of course.