Document initial api

The plan is to document the initial api methods and use the PR to get everyone's feedback on how the api should look before starting on the implementation.
We are basically going to use graphite's [`summarize`](http://graphite.readthedocs.org/en/latest/functions.html#graphite.render.functions.summarize) under the hood.

One alterntive would be to let people decide on the graphite functions themselves, in which case the metric names in the request would be allowed to look something like `summarize(stores.a.b.last, "1hour", "last")`. It does mean parsing out the name, but we'd probably need to do that anyways to get the aggregation method if we did the graphite summarize query construction ourselves.