Log request metrics to a collectd instance

Implements logging of rendr.it metrics to a collectd daemon over UDP. 

* Includes `pycollectd`, a Python implementation of `collectd`'s [binary protocol](https://collectd.org/wiki/index.php/Binary_protocol), built on top of Tornado's ioloop
* Implements a `CollectdLoggingApplication` class to allow logging of metrics by Tornado web applications by overwriting `log_request()`
* Adds a `--collectd-server` flag to `bin/server` to specify the address of the `collectd` daemon to log to