Html5 fullscreen api

Implements a new fullscreen api, which provides helpers for users to control aspect ratio and canvas resizing in fullscreen mode, and to get consistent cross-browser behavior. Resolves issue #2556.

Two demos live:

  1. A test build of my Cube sample: https://dl.dropboxusercontent.com/u/40949268/emcc/test_html5_fullscreen/Cube_fs.html
  2. A build of the html5 test with interactive buttons for testing: https://dl.dropboxusercontent.com/u/40949268/emcc/test_html5_fullscreen/test_html5_fullscreen.html

what's a "letterbox"?

also, comment seems to be missing 'in a' before "cross-browser"