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 
A sidenote (applies to various places in the PR) - when you are in a promise handler function (`then` or `fail`), there's no need to explicitly return a promise. _Anything_ you return will be wrapped in a promise. So just calling `return` is the same as calling `return Q()`, the latter just does some unnecessary extra work.