Added an optional callback for the ping function

m.ping(yourCallbackFn) will call yourCallbackFn(m.pingTime) once it is
resolved.
Update README.md to reflect changed to mutlplayer.js

Update README.md to reflect changed to mutlplayer.js made in commit cc70e4334b9c4e1c852e4a5209c75096ea4ce5be
Merge pull request #1 from dudeofawesome/master

Added an optional callback for the ping function