Adding RRD handler.

This allows us to store Diamond data in RRD files.
RRD is a general purpose format for storing time-based
data and allows for aggregation to efficiently track
data at different resolutions.

Signed-off-by: Adin Scannell <adin@scannell.ca>
Merge pull request #429 from amscanne/add_rrdtool_handler

Add handler for generating RRD files.