Adds support for "prefix" parameter in Ganglia reporter. 

The following changes are made:

- Pull up the implementation of the "prefix" parameter from the GraphiteReporterConfig
  into the AbstractHostPortReporterConfig so that it can be shared across the
  graphite and ganglia reporters.
- Optional support for metrix prefixes in the Ganglia reporter. This feature is only
  available if you include as a dependency a fork of the metrics project that supports this feature,
  such as http://github.com/mspiegel/metrics. The vanilla 2.2.0 GangliaReporter does not have support
  for metric prefixes although this feature has been added in version 3.x of the
  library.
- In the MetricPrefixGangliaReporter fixed a bug where empty group prefixes would
  be appended onto the group name as "_". Now only non-empty group prefixes
  are appended.
- In GangliaReporterConfig applied the "prefix" macro substitution options to
  the "groupPrefix" parameter in case someone would like to use them.
- Updated README.mdown to reflect that "prefix" parameter is available in both
  graphite and ganglia reporter configs.