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 
I guess we'll want to replace that with "pivotaltracker". It's what the `debug` log module uses for filtering (so you can set the `DEBUG` env var, e.g. `DEBUG="pivotaltracker"` if you're only interested in PT logs (it's actually a regexp so you could do `DEBUG="app:pivotal:*"` or whatever) and it's also visible in the log lines.