Added Songkick Spice plugin.

This adds a Spice plugin for the [Songkick API](http://www.songkick.com/developer/). It answers 2 types of queries:

* artists like _artist-of-your-choice_
* concerts in _city-of-your-choice_

This requires 4 separate APIs:

* We get an artist's id via http://www.songkick.com/developer/artist-search.
* We get artists similar to that artist_id via https://www.songkick.com/developer/similar-artist.
* We get a city's metro area id via http://www.songkick.com/developer/location-search.
* We get events in that metro_area_id via http://www.songkick.com/developer/upcoming-events-for-metro-area.

The *artist_id* and *metro_area_id* calls are done in the .pm files. The results are used to make a client-side JSONP request for the data to be presented to the user. This creates a situation where the api key must be shared with the client. Is this permissible?