Use Konacha for testing JavaScript

As previously discussed on the mailing list.

One concern raised there was that we needed to have the ability to
run specs from the command line without opening a browser window.
I've configured konacha to use poltergeist, a PhantomJS-based driver,
so the `konacha:run` rake task is completely headless.

We seem to have general consensus that testing the OSM.org JS would
be a good thing. What needs to happen to adopt konacha (or any other
technology)?