Minor changes on graphite plugin.

- Added metrics_format option to define the resulting format of the
  metric string that will be sent to graphite.

  example:

    graphite {
      host => "graphite.local"
      port => 2003
      fields_are_metrics => true
      include_metrics => ["foo"]
      metrics_format => "my.system.%{metric}"
    }

- added test suite for graphite plugin