Add etcd support with self and store metrics.

# What this patch does

This patch adds a check for etcd. It fetches metrics from etcd's `/v2/metrics/self` and `/v2/metrics/store`. I ripped most of the code form the existing Mesos check.

# Motivation

We use etcd in production and would love to have Datadog visibility into it's work. In addition to the various gauges and rates it also turns the 'leader: true` value in to a 1 or 0 and creates a metric. This way we can watch for leadership changes and split brains.

# Testing

I'm running this check on a staging machine and seeing metrics!

![screenshot 2014-12-07 11 18 04](https://cloud.githubusercontent.com/assets/45159/5332206/c15bc0ca-7e02-11e4-9922-e7d39fa9d8d3.png)

Thanks for explaining, I only looked quickly at some etcd doc and plugins (and saw only .store and .leader namespaces) and I have used it just a few times, so definitely not an expert on the topic. If these names make sense to an etcd guru please keep them :wink: