Basic monitoring support for Apache Mesos

This is a basic agent check for [Apache Mesos](http://mesos.apache.org/) that consumes Mesos REST API endpoints and emits Datadog metrics.  We've been using this code successfully with the latest Mesos 0.18.0 release and it's proved sufficient to build some useful dashboards.  

Mesos currently provides statistics from three endpoints which are translated into Datadog metrics:
* `/master/roles.json`:  list of running frameworks and resources being managed
* `/master/stats.json`: aggregated statistics about # of slaves, tasks, offers, etc
* `/master/state.json`: detailed information about frameworks, tasks, resources, etc.
* link to the (endpoint source in Mesos](https://github.com/apache/mesos/blob/85559cc96fea471ecf9ad0de4dbc19d038854940/src/master/http.cpp)

Hopefully others running Mesos will find this useful.