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 
Nit: I suggest we move those test strings somewhere (either a fixture module or leave them in this module but put them at the top a single var). We're reusing the same string all over again so I think it makes sense to keep it in a single place.