Adds support for "prefix" parameter in Ganglia reporter.

[mspiegel]

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.

[mspiegel]

Actually I'm going to close this request. I'm going to move most of the implementation into my fork of the metrics library and then make this feature optionally available in reporter-config via reflection.