Began Sprint2 Demo

* Modified AltMenuScreen and ButtonPanel to take three Runnables instead
of one, to launch single and host/join multiplayer games.
* Changed up some variable names and wording to make 'hosting' more
clear
* Modified UI Test Application to use multiple buttons, which currently
do nothing (for that test only)
* Created sprint 2 demo runner, and 3 apps which launch different game
behavior (single player, hosting, and joining multiplayer)
Fixed bug in ButtonPanel

Start button was running two actions for every clip because of a
copy-paste error. Whoops!
Added IP Popup Window to Sprint 2 Demo

* Allows Server IP Entry
* Allows re-try of IP entry, if unable to join server (throws an error,
which is ugly, but functional
* Correctly closes IP window if another option is selected
Added music to demo

* Added another music file to the game (may change later!)
* Added music to the demo, which should start on single or multiplayer
game start.
Merge pull request #285 from BU-CS673/Aaron

Aaron -> Canfield