Tfmesos

- [86] Add heartbeat for pymesos

- [85] Add config for env

- [82] tfrun can bring all workers' log back

- [74] Retry when tasks failed
- [77] Add `initializer` and `finalizer` command for `tfrun`

Thanks treadstone90 for these marvellous contributions.

- [63] Run master worker on Mesos when using tfrun
- [69] Added force_pull_image support.
- [72] Add option for mesos framework name.

- [63] Run master worker on Mesos when using `tfrun`