Adding gzipped-XML-tree support to gmond, gmetad, and gmetad-python

This patch does the following:
- Adds a command-line parameter for gmond, "-z", which causes it to
  emit its output XML trees as a gzip stream instead of as plaintext
- Changes gmetad and gmetad-python to automatically detect a
  gzipped data stream from a gmond and handle it appropriately