Add support for executor authentication

[atringham]

At the moment there is no way to enable authentication for the executor using pymesos.

When executor authentication is enabled the agent injects MESOS_EXECUTOR_AUTHENTICATION_TOKEN into the the executors environment. This needs to be added to each requests header in order to authenticate, like the following

Authorization: Bearer MESOS_EXECUTOR_AUTHENTICATION_TOKEN

See http://mesos.apache.org/documentation/latest/authentication/

If you think this is an acceptable approach I would be happy to provide a PR

[ariesdevil]

PR is welcome.