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.