FIX: Caching Bug

/api/current_match was being cached. Not by AngularJS, but by the
browser. The culprit was Cache-Control: it was set to private even for
/api/* routes. Added an api-specific before filter and changed the
default one to act as a 'fallthrough' before filter.

App.js also received some tweaks. I incorrectly thought that AngularJS's
$http service had busted helper methods, but it wasn't the case.