Specify 1:n communication between a PresentationSession and multiple connected PresentationSession

[markafoltz]

In some scenarios the presented page may have active connections to multiple PresentationSessions on other pages. In a gaming scenario, each connected session would represent a game player. Some information (like dealing a card into the hand) should be sent to one player, while other information (playing a card face up) should be sent to all players.

We need to specify:
(1) How the presented page may communicate to a specific connected page
(2) How the presented page may broadcast - communicate to all specific connected pages

[markafoltz]

Note the following thread discussing some concrete spec changes to facilitate this case:

http://lists.w3.org/Archives/Public/public-secondscreen/2014Nov/0047.html

[anssiko]

Made this P3 per @mfoltzgoogle's input at http://lists.w3.org/Archives/Public/public-webscreens/2014Dec/0011.html.

[markafoltz]

I am merging this into #19, since I think the two behaviors (connection/disconnection of multiple sessions and messaging among them) will be hard to separate in the specification.