Exposed constructors for types declared in Log.java.

This allows java API users to construct objects in tests without extra
hoop-jumping.

This patch also fixed some javadoc errors.

Review: https://reviews.apache.org/r/22837
Updates PoweredByMesos documentation.
Fixed a style warning.
Modified two slave recovery tests to deal with the cases where 'update'
in an isolator launches a subprocess.

Review: https://reviews.apache.org/r/22900
Temporarily disabled health check tests due to MESOS-1533.
Properly disable health check tests.
Added support for following redirects to the fetcher (via stout/net).

I've added a call to set CURLOPT_FOLLOWLOCATION to true, which allows
the fetcher (when using http[s]://) to follow 30X redirects when
fetching.

Review: https://reviews.apache.org/r/22675
Explicitly terminated scheduler process in ~MesosSchedulerDriver().

Without this fix, Mesos tests that have assert failures get deadlocked
on ~MesosSchedulerDriver() (because driver.stop() is not called)
waiting forever for the scheduler process to exit. This causes the
tests to never exit.

Review: https://reviews.apache.org/r/22923
Added a test to verify that frameworks with equal share get equal
number of allocations.

Review: https://reviews.apache.org/r/22714
Wait on orphan cgroup destroy during launcher recovery.

Review: https://reviews.apache.org/r/22893
Fixed FrameworkReregistersFirst and SlaveReregistersFirst allocator tests.

Review: https://reviews.apache.org/r/22945
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.
Add environment test for docker