Provide the option to remove unnecessary XML data.

The EXTRA_DATA and EXTRA_ELEMENT data returned by Ganglia is unused in many code paths, so by stripping it we can see XML parsing performance gains of about 50% for large data sets.

In production, gmetad was returning 26MB of XML data for the everything path.  By filtering this out, I took parsing time from an average of 2.1s to 1.1s, a nearly 50% improvement.