Adds mlab_export.py for serializing rrd data.
Adds mlab_export.py for serializing rrd data.

This change adds an export script that would be run periodically to
serialize portions of the RRD data for collection by the data collection
pipeline.

stephen-soltesz/collectd-mlab,m-lab/collectd-mlab,m-lab/collectd-mlab,stephen-soltesz/collectd-mlab