Support for units

[fhats]

We currently commit the cardinal sin of graphing by not including units on our y-axis. I'm not sure how we'd implement this, since we primarily consume RRDs and I don't think rrdtool provides a way of storing units in RRDs.

Still, this would be nice to have.

[bigo]

the way I'd always imagined this was that [SUPER SECRET TECHNOLOGY 1] would emit them as part of the initial "key", and then [SUPER SECRET TECHNOLOGY 2] would aggregate+write stats to disk as though that were just part of their name. Then the DataSource could transparently strip that component of the key, and communicate the units value out to the front end during the list_path() or data() operations. That is to say, make communicating units from a DataSource to the Firefly front-end a standard thing, then hack together whatever you have to to get units from the various datastores