Add support for keymessage event.

- Added listener for 'webkitkeymessage'.
- Changed the algorithm of initData and e.message to follow v0.1
  of the Encrypted Media Extensions.
- Added call to generateKeyMessage().
- Pass the sessionId from the keymessage event to addKey().
- Added doKeyMessage function to handle the keymessage event.

Change-Id: I8c6601dcda497b44691c7f770bc73d5e14fd7563