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.
Should "source or get_hostname()" (repeated at line 375 below but without passing self.config in) be pulled into a common location so that future developers incorporate it? Or is it not a common enough case?