Docker bridge

Fixes #587 (Docker bridged networking mode)

For initial review.

- [x] Update docs
- [x] Update Mesos dependency once version 0.20.1 is released
- [x] Do dynamic global port assignments
- [X] Verify `ports` attribute works as expected
- [X] Publish and update dependency of mesos-utils for 0.20.1
Good call, will modify this to surface any throwables wrapped by the `Try` -- in fact, will probably change the method signature to just return the `Try`.