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
Only catch exceptions when proxying, let container handle local exceptions
Block on API requests and improve error handling

- 404 when trying to stop/scale a non-existing app
- 422 when trying to start an already existing app
- 504 on timeouts
Merge pull request #68 from mesosphere/blocking-api

Blocking API