Added initial draft of contributor guidelines.
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.
Refactored health check subsystem.

- Moved task status update logic to
  MarathonHealthCheckManager, refactored to
  delegate state to the HealthCheckActor
- Renamed DelegatingHealthCheckManagerTest to
  MarathonHealthCheckManagerTest
- Added EventFilters to health check unit tests.
- Deleted DelegatingHealthCheckManager
- Dropped the --executor_health_checks flag
- Removed --executor_health_checks from flags doc
- Updated the health checks doc
- Updated the REST doc
- Updated bridge net example to include cmd check
- Fixes #664
- Fixes #724
Fixed error in post app json
Fixed up REST docs on command health checks.
Cleaned up handling of cmd checks in TaskBuilder.
Merge pull request #728 from lukaswelte/patch-1

Fixed error in post app json
Merge pull request #727 from mesosphere/724-health-checks-refactor

Refactored health check subsystem.
Cleaned up examples, added Getting Started headline
Added note about regression tests.
Replace random service ports in docker mappings.

- Fixes #714
Merge pull request #715 from mesosphere/contributor-guidelines

Added initial draft of contributor guidelines.
GroupManager: edit for clarity, rm unnecessary ids
Merge pull request #735 from mesosphere/714-bridge-random-service-ports

Replace random service ports in docker mappings.
Added ability to handle CentOS/RHEL systems.
Turn on HAProxy service for CentOS/RHEL.
Add exit for when an unsupported OS is found.
Fixed my indents. Replaced haproxy.cfg with a variable.
Update download links.