Add `hostname` support for `GROUP_BY` constraint
Fix performance issue with `GROUP_BY` constraint

Assume there are three hosts. Host1 has 2 tasks, while host2 and host3
both have 1 task. We should assign tasks to either host2 or host3, but
in the original implemetation, it waits for only one of the two hosts.

This commit also simplified the implementation, all tests passed.
Handle containers without port mappings.

- Fixes #779.
Merge pull request #781 from mesosphere/779-container-dropped-no-mappings

Handle containers without port mappings.
Minor scaladoc fixes.
Merge pull request #782 from mesosphere/scaladoc-fixes

Scaladoc fixes
Setting version to 0.7.5
Setting version to 0.7.6-SNAPSHOT
Update docs for release 0.7.5
Merge pull request #783 from mesosphere/docs-update-0.7.5

Update docs for release 0.7.5
no message
Merge pull request #784 from bolcom/used-by-bolcom

Add bol.com to list of companies using Marathon
Update README.md
fixes #788 - Emit events for registration, reregistration, disconnected
Upgrade to mesos-utils-0.21.0-1
Merge pull request #794 from mesosphere/mesos-utils-0.21.0-1

Upgrade to mesos-utils-0.21.0-1
Added privileged, parameters to DockerInfo proto
added tests for scheduler events
Merge pull request #792 from mesosphere/wip-scheduler-events-drexin

fixes #788 - Emit events for registration, reregistration, disconnected
fixes #790 - version field in AppUpdate has to be exclusive