Several fixes and some code cleanup.

Fixed API Caching from handling non-XML responses as XML and throwing a
fatal error.
Added a way around secondary copies of jQuery from overriding the
Challonge plugin.
Fixed score reporting when scoring is on second setting and opponent
scoring is disabled and the participant is matched with a opponent that
signed up via other means.
Fixed a minor security issue.
Fixed the Signup button actions for unregistered users.
Added some CSS to prevent twentyeleven theme header from showing over
thickboxes.
Some general code cleanup.