RFC: Add Librato sink

This adds an experimental sink for publishing statsite metrics to Librato. The sink is implemented in Python using the ASCII protocol (modeled after the Graphite sink). The sink takes a single configuration file as an argument that contains the Librato account credentials as well as a few metric processing configuration options. The configuration options are modeled after the [Librato statsd backend](https://github.com/librato/statsd-librato-backend) and the publishing code comes from the [Librato collectd plugin](https://github.com/librato/collectd-librato).

Is there an established policy for adding new sinks to statsite? Maintaining them in-repo would mean that protocol changes would be kept in-sync. However at some scale it would become impractical to have to support multiple different sinks across statsite releases.