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 
Oh, one exception is when you want to signal failure. Then you have two options. Either `Q.reject(...errror...)` or `throw new Error(...)` (exceptions are converted to a rejected promise by the promises framework).