Added plugin for NRK.

Adds support for tv.nrk.no, NRK's streaming service. Works for both live and aired shows.

Live: ```livestreamer http://tv.nrk.no/direkte/nrk1 best```

Other: ```livestreamer http://tv.nrk.no/serie/nytt-paa-nytt/muhh29000714/21-02-2014 best```

(Streams may or may not be available from outside Norway due to copyright restrictions)