Add stats-meta-data mappers

Hi,

I've had the desire to be able to rename my metrics dynamically (and also change the group they're put into).

So I've implemented a middleware for the plugin which lets you write a JavaScript method which maps the metric's meta data to another one.

Also I've introduced a switch which enables/disables the debugging output of the Ganglia plugin so you can switch debugging on but disable the Ganglia plugin debugging output.

Real used example config (statsd_config.js):
```javascript
....
, ganglia: {
    host: 'xxx.xxx.nu',
    port: 8650,
    useHost: 'statsd',
    spoof: 'StatsD:StatsD',
    group: 'StatsD',
    debug: false,
    mappings: {
      ALL: {
        mappingMethod: 'mapMetric',
        mappingSource: './ganglia_mapper'
      }
    }
  }
```

./backends/ganglia_mapper.js:
```javascript
exports.mapMetric = function (metric) {
  // Remove the first part of the metric path and change the units & group
  if (metric.name.indexOf('stats_counts_myproject.') != -1 || metric.name.indexOf('stats_myproject.') != -1) {
    metric.name = metric.name.replace('stats_counts_myproject.', '')
    metric.name = metric.name.replace('stats_myproject.', '')
    metric.group = 'myproject'
  } else if (metric.name.indexOf('stats_timers_myproject.') != -1) {
    metric.name = metric.name.replace('stats_timers_myproject.', '')
    metric.units = 'milliseconds'
    metric.group = 'myproject'
  } else if (metric.name.indexOf('stats_gauges_myproject.') != -1) {
    metric.name = metric.name.replace('stats_gauges_myproject.', '')
    metric.units = 'gauge'
    metric.type = 'float'
    metric.group = 'myproject'
  }

  // Remove eventually leftover first parts of the metric path
  metric.name = metric.name.replace('stats_myproject.', '')

  if (metric.name.indexOf('statsd') == -1) {
    // Generate a new metric group based on the metric path
    var nameParts = metric.name.split('.')
    if (nameParts != null && nameParts.length > 0) {
      metric.group = metric.group + " - " + nameParts[0] + ":" + nameParts[1]
    }
  }

  return metric
}
```