Category from Feeds

[erictchiu]

Add support of RSS/Atom categories in FoF.

Which RSS schema to use:
http://www.thearchitect.co.uk/schemas/rss-2_0.xsd
http://dublincore.org/2010/10/11/dcelements.rdf#

Simplepie code can parse categories. Update database script and FoF for supporting categories.

1. extend fof_item and fof_feed tables to add the category field
   from RSS/Atom.
2. extend the user interface to display category, with add/modify/delete ability similar to tag functionality we currently have.

[erictchiu]

Using category from feeds will probably involve manual data cleaning before displaying it, so I prefer to use existing FoF tagging for now rather than categories from feeds.