Use webfinger.js

I split out the webfinger logic from remotestorage.js and replaced it with using the `webfinger.js` library. All of the functionality works fine, the one issue I have is I can't get the tests to pass. They are doing some `XMLHttpRequest` hijacking it seems, but for some reason it's not finding anything. 

If you compare how `webfinger.js` makes the HTTP calls [1] and how the old implementation in `dicover.js` did [2], they are almost exactly the same, so I'm not sure what's going on. Anyone have any ideas here / experience with writting http request stubbing? If anyone want's to take a look I'd appreciate a second pair of eyes on it.

[1] https://github.com/silverbucket/webfinger.js/blob/2cedc3c1b3923b43f40532753a091736278a64d7/src/webfinger.js#L89-L126
[2] https://github.com/remotestorage/remotestorage.js/blob/ebb1d6c16705b1ee21d910a74fbe04b7f3dd11f6/src/discover.js#L38-L49
Ah right, I just realized it's all just using global/window anyway.

So that means we should do the same with Bluebird, right?