Librato metrics support

This pull request brings support for [Librato Metrics](http://metrics.librato.com). Currently it replaces the Graphite functionality entirely, but the goal is to extract both ways to fetch metrics as a sort of pluggable transport.

Notable distinctions between both ways of fetching metrics are mostly that Graphite supports fetching multiple metrics at once and Librato Metrics doesn't. The code has changed around that a bit.

The implementation has moved from multi-dimensional arrays towards using hashes indexed by metric name instead.

Note that this is a work in progress.

Discuss!