Implemented thumbnail rendering for ReStructuredText (.rst) 
and MarkDown (.md) file attachment types. The rendered 
thumbnails are displayed as HTML scaled down by css (instead
of rendered as image thumbs), and are mem cached. 

Also changed the way MimetypeHandlers are registered to match
existing infrastructure for registering Review UIs, replacing
the old way of doing it through __subclass__

1. Ran unit tests for reviewboard.attachments.tests:

- Updated tests now all pass after adding setUp() and tearDown()
  to register / unregister the test Mimetype Handlers.

2. Manual testing on localhost:

Tested by uploading to a new review-request:
- raw .txt file
- raw .rst file
- raw .md file
- raw .jpg file
- raw .rst file with javascript injection
- raw .md file with javascript injection

Visual inspection of the rendered thumbnails all pass: 
- Thumbnails appear immediately (without a refresh), reflecting
  latest feature updates to master.
- Scaling, cropping and rendering all appear to be correct.
- Javascript injection correctly escaped for malicious
  .rst and .md files

See updated screenshots (2012-11-24, in the files attachment section)