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'm not sure I get this. You're mapping the labels to `{id: label.id}` and then pushing the `reviewed` label. Why do you need to map them? Couldn't you just push {name: 'reviewed'} to `story.labels` and send that?