ReviewBoard Extensions can now define how to create thumbnails
for specified file attachment mimetypes.

Added FileAttachmentThumbnailHook to generalize a hook point for
all files of all mimetypes. Mimetype Handlers for text-based
mimetypes can leverage `reviewboard.attachments.mimetypes.TextMimetype`
by simply overriding `_generate_preview_html` to define how to
generate the HTML used in the thumbnail, instead of having to
redefine `get_thumbnail`.

Test Steps:

- Disabled the registration of XMLMimetype in reviewboard.attachments.__init__ (
- Verified that XML thumbnails now fall back to raw uncolored text)
- Copied over the XMLReviewUIExtension from the sample extension branch, and:
  - Added XMLThumbnail.py to define the XMLMimetype class for generating XML thumbnails
  - Modified extensions.py to use the FileAttachmentThumbnailHook to hook XMLMimetype into ReviewBoard to handle XML thumbnails.
- Verified that the Extension installs without error
- Verified that Extension-installed XML thumbnails feature work exactly as did before.

(see attached screenshot, disregard the hyperlink lines - that was due to a separate change to master: http://reviews.reviewboard.org/r/3569/)