Categories

Initial implementation of categories.

Here's how they would work:

1) To file a post in a category, add a "category" metadata field to it.
2) You will get a list of your categories in the new "Tags and Categories" page (formerly just tags)
3) There are RSS feeds for categories, post lists, etc.

Caveats:

* You can't have categories and tags with the same name (should add something that detects it?)
* If you don't categorize a post, it has a category of '' and is ignored everywhere categories are used
* If all your posts are uncategorized, this should produce pretty much the same output as before