New peer2peer UI

The UI is loosely based on the other webrtc demos. The biggest improvement is to be able to use on mobile screens and also editing constraints should be easier.

I've tested the files against grunt htmlhint, jscs, csslint, jshint and only one warning remains:
[L207:C12]
WARNING: Heading (h2) should not be qualified. Headings should not be qualified (namespaced). (qualified-headings) 

I will fix that a later date if no-one objects. I did some general cleanup of adjacent files as well, such as putting the doctype in the right place etc. I also renamed stylesheet.css to main.css to follow the structure of the other demos.

The page is currently served [here](http://kaptenjansson.github.io/webrtc/samples/web/content/manual-test/peer2peer/) for testing.