(EventView) Use FirebaseCollection to handle list of comments

See commit c86fcf0e85b0b22acce6369b2f0fdb809a41f3da for more info. I added a
numItems() function to the FirebaseCollection object that will hopefully be
pulled upstream into the official angularFire project. In any case, we'll keep
it here.
(EventView) Add sorting options for comments

We added a 'date' property to all new comments so they can be sorted
accordingly. It's important to know that adding a negative symbol in front of
any orderBy predicate will reverse the order of it. See the AngularJS docs for
more info on that: http://docs.angularjs.org/api/ng.filter:orderBy

Also, we're including a sort by "Best" option for the future, when we add some
point system to the comments (kudos, upvotes, likes, etc.).