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
Can we do the same with bluebird? How does the code actually get loaded when running remoteStorage.js just from the src files? Not sure we need to keep supporting running it like that, but I know that at least @michielbdejong used it like that.