Refactor configuring from XML file

This refactoring occurs in 2 steps:
1. Moving `MBeanHolder` and `MBeanAttribute` to their own files, this exposes them for testing
2. Separating XML configuration for Ganglia and JMXetric into their own classes.

Overall the this led to increase in code base, but many of them are javadoc, and new methods. What we gain is the ability to test configuring from XML files, and also the separation of configuring Ganglia and JMXetric. Classes are *smaller* because they do less (even though the number of lines increased), and methods are also smaller.