Added support for the optional game ID in server info

Although this is (almost?) always equal to the application ID it
might be useful in the future.
Simplified server info packets
Adapted GameServer tests to recent packet changes
Simplified some parts of SteamId
Improved error handling for XML data
Also test on Ruby 2.0.0
Depend raise minimal multi_xml version to 0.5.2

See CVE-2013-0175
Streamlined parsing of some game stats
Version bump to 1.3.3
Merge branch '1.3-stable'