Update mesos dependency to 0.20.0
Support Docker and ContainerInfo.

* Create ExternalContainerInfo and ContainerInfo classes in
  MesosSlaveInfo to better map with the Mesos API.
    - ExternalContainerInfo can be used by external containerizers
    - ContainerInfo is to be used with the new containerizer system
      (only DOCKER is supported right now)
* Add ability to specify Volumes
* Refactor a bit the config parts to use the MesosCloud and
  MesosSlaveInfo objects everywhere.
* Use MESOS_SANDBOX environement variable
  Fallback to './' if not present. This should work for docker and
  non-docker situation to run the slave.jar file.
Updated README with information about native docker support and
authentication with Jenkins master.

Fixes #40, fixes #51, and fixes #55.
Fixed Volumes UI bug for Docker Containerizer.

fixes #58.
Updated README.md.
Additional URIs configuration

This is especially useful if you want to use a private Docker registry.
Add help information for Additional URIs config.
Merge pull request #59 from criteo/add_uris

Added support for additional URIs configuration.
Added listener to print mesos slave hostname in console output
Merge pull request #61 from maselvaraj/master

Added run listener to print mesos slave hostname in console output.
[maven-release-plugin] prepare release mesos-0.5.0
[maven-release-plugin] prepare for next development iteration
addressed review comments
Merge branch 'master' of https://github.com/jenkinsci/mesos-plugin