Added initial draft of contributor guidelines.
Minor refactoring for framework authentication

- Adds unit tests for the config parser.
- Less side effects in MarathonSchedulerDriver.
- Better exception msg if secret can't be read.
- Naming cleanup in MarathonSchedulerDriver.
Merge pull request #723 from mesosphere/auth-cleanup

Minor refactoring for framework authentication
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.