Adding collectors for Aurora and Mesos.

The Aurora collector collects from both the /vars.json endpoint and from the
/jobSummary REST API endpoint.

The Mesos collector collects both master and slave metrics as published by
/metrics/snapshot.
I think we maintain a fork for now. There's a bigger refactoring that I'd like to do in order to get this submitted upstream. I'm maintaining the collectors in a way that's compatible with their standards, so it should be easy to push them as well.