Extended RetroPlayerInput to provide multiple player support

Added the ability to monitor up to 4 controllers (and extendable via
define)

Tested with up to 4 controllers, and inputs are all reconignzed.  (Only
tested with 4 XBOX controllers on a single wireless adaptor)

Bug fix based on garbears comments.

Removed const, removed GameClient.cpp update(mistakenly added)
Merge pull request #2 from AndrewButt/retroplayer

Extended RetroPlayerInput to provide multiple player support