Make the Ganglia Input plugin work and be testable.

Added a new unit test for the Ganglia plugin and verified that it works both in a test fixture and in my employers clusters.

Added a lot more documentation to make it easier for others to use the plugin and know what fields are available and added external references to help people dig deeper.

Verified in 1.2.2, but I was having problems building the mysql2 gem in master, so I was unable to test there.