Add servicePort to port mapping object.

- Vendored Mesos ContainerInfo proto for extension
- Added servicePort to PortMapping proto
- Regenerated protobuf classes
- Added AppDefinition.servicePorts() method
- Modified tests as necessary
- Updated docs for new portMapping parameter
- Fixes #672
Added info about bridged networking defaults
Merge pull request #677 from mesosphere/672-bridge-service-port

Add servicePort to port mapping object.