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.
- Copy/pasted the com.yammer.metrics.reporting.GangliaReporter class into the
  MetricPrefixGangliaReporter class. In MetricPrefixGangliaReporter implemented support
  for optional metric prefixes. The original GangliaReporter does not have support
  for metric prefixes although this feature has been added in version 3.x of the
  library. The GangliaReporter is under the Apache License 2.0.
- 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.