Add support for summaries

This change adds supports for summaries, which accumlate sample values
and return a set of quantile values (0.5, 0.9, 0.99). A couple of
changes and cleanup were necessary, as Summaries are more complex values
than a simpler counter or gauge.

@porras / @matttproud: Couldn't sleep in the plane, so I went ahead :) You might also want to have a look at the output of the updated included example rack app: https://gist.github.com/grobie/7331513