Add support for downloading patches and parsed diff data in the API.

This adds support for new mimetypes to FileDiffResource and DiffSetResource.
Both now support the text/x-patch mimetype, and will return the stored
patch. In the case of FileDiffResource, it's the patch for that particular
file, and for DiffSetResource, it's the patch for all files in the diff.

FileDiffResource also supports new
application/vnd.reviewboard.org.diff.data+json and
application/vnd.reviewboard.org.diff.data+xml mimetypes, which return
the parsed diff data in JSON or XML formats. This includes the
insert/delete/replace lines, move information, and more. It also supports a
?syntax-highlighting=1 parameter for returning data in a form that can
have CSS rules applied for syntax highlighting.

This new diff data resource should make it possible for third party
applications to render diffs without reimplementing Review Board's
diff viewer logic.

Tested each resource using curl in order to see the various forms of output.