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.
Bug fix. Optional prefix to Ganglia metrics should work with either original or modified metrics-core dependency.
Merge pull request #8 from mspiegel/master

Adds support for "prefix" parameter in Ganglia reporter.