Add test for unique host and balanced cluster constraints
Bumped scala lib versions.
Fixed constraint clustering.
Use pom.xml to define marathon version.
Fixed task state serialization.

Rather than serializing several messages separately, serialize one
message per app to/from the state handler.
Fixed a bug in fetching task state.
Updated according to Tobi's comments.

Cleaned up code, default GROUP_BY operator to a value of 0 if no value
specified.
Merge pull request #64 from mesosphere/cluster-contraint-fix

Cluster contraint fix