Fixed a regression in ExecutorInfoChecker to allow an executor with the same id but different executor info as long as it is on a different slave.

Review: https://reviews.apache.org/r/23187
Added the low level scheduler example using libprocess.

Review: https://reviews.apache.org/r/22539
Fixed flaky OrphanTasks test.

Added Clock::settle after StartMaster() to ensure the master finishes executing _recover().

Review: https://reviews.apache.org/r/23001
Made sure Cluster::Masters::start() returns when the Master is ready for incoming requests. i.e., after it has finished executing _recover().

Review: https://reviews.apache.org/r/22857
Fixed and renamed AllocatorZooKeeper tests.

Review: https://reviews.apache.org/r/23220
Used shared bind mount to fix MESOS-1558.

Review: https://reviews.apache.org/r/23246
Minor fix to SlaveReregistersFirst and FrameworkReregistersFirst
allocator tests.
Updated Getting Started page for 0.19.0.
Fixed some Javadoc errors.
Improved gate.hpp documentation.

Review: https://reviews.apache.org/r/18988
Created an example LoadGeneratorScheduler to test Master's framework rate limiting feature.

Review: https://reviews.apache.org/r/23250
PortMapping: allow containers to recover even when they were not
managed by Network Isolator previously.

This eases deployment where a slave could be upgraded to use Network
Isolator without removing all the existing tasks.

- Added a new test.
- Moved all portmapping tests to a new file.

Review: https://reviews.apache.org/r/23214
Minor fix to mesos_tests_SOURCES in Makefile.am
Added 'bool' return value to Containerizer::launch.
Added composing containerizer and --containerizers flag.
Added a 'Docker' abstraction.
Added a Docker containerizer.
Bug fixes and cleanups in Docker abstraction.
Integrated DockerContainerizer.
Updated an example to use ContainerInfo to run Docker.