Add Metrics

* Adds `-metrics` command line.  This can take a comma separated list of metrics outputs. Currently supports `console` and `graphite`.  Over time I think these should be more URL like, so you can have remote IPs/ports, but I think in general we need a more in depth configuration strategy.
* Adds dependency on https://github.com/rcrowley/go-metrics
* Downsides: Uses locks around:
 * Getting Upstream-specific metrics handlers
 * Getting all HTTP Response codes.
* I'm voting lets get it done, and then benchmark/profile separately rather than worry about it too much right now.