Change fish-pepper to use promises

[rhuss]

We already have an async, promise based lib (nodegit, but see #3). Currently we need to use node-fibers for waiting on a promise fullfillment, which is a classical 'future'. However, since JavaScript is single threaded, this can not be easily done in plain JavaScript because the single thread will wait until a function returns (so you can't wait within a function on the result of an asnc operation, in fact even start the async operation won't even start). Therefore node fibers, an V8 extension, can be used to allow for multi-threading an hence waiting on promise from within a method.

But this comes with a cost: Installation of fibers takes its time, making everything more 'heavy weight'. Also, promises and callbacks are really more in the 'nature of JavaScript', so we shouldn't work against it (even not in a batch script like fish-pepper).