Implement the issue tracker interface for Pivotal Tracker

Here we go. Feel free to tell me off, I haven't done too much JS/CS, so there might be some noob mistakes/not entirely optimal solutions out there.

@realyze 
Would be safer to test for `>= 0`.

Oh and a hint: cool guys do `~line.indexOf(prefix)` instead of `>=0` (`~` is bitwise NOT so `~-1 == 0).