Fixed MESOS-1427: 'friend' statement in Variable resolved to a different State class on clang 3.2

Review: https://reviews.apache.org/r/22054
Adds Virdata and Yieldbot to #PoweredByMesos page.
Fixed SchedulerTest.MetricsEndpoint test.

Review: https://reviews.apache.org/r/22089
Hid "acls" flag from master's flags.
Updated Getting Started doc.

Review: https://reviews.apache.org/r/21964
Updated the CHANGELOG for 0.19.0.

Review: https://reviews.apache.org/r/22099
Updated the upgrades document for 0.19.0.

Review: https://reviews.apache.org/r/22100
Improve log message for slave moving itself into a cgroup.

Review: https://reviews.apache.org/r/22047
Add helper in tests/mesos.hpp to create task with resources

Review: https://reviews.apache.org/r/22048
Ignore slave cgroup during isolator recovery.

Do not consider the slave cgroup (from --slave_subsystems) as an orphan
during recover().
Added process ID prefixes for ZooKeeper related libprocess Processes which are helpful for debugging.

Review: https://reviews.apache.org/r/22161
Fixed memory leaks and cleaned up the mesos containerizer logic.

Review: https://reviews.apache.org/r/22159
Updated dropped message logging to use VLOG.

Review: https://reviews.apache.org/r/22164
Added a concurrent 'Queue' to libprocess.

Review: https://reviews.apache.org/r/20308
Introduced a low-level scheduler API.

Review: https://reviews.apache.org/r/20309
Removed unsignedness from constant.
Removed newline (style failure).
Increased the registry store timeout for tests.
Fixed a segfault in libprocess.

Review: https://reviews.apache.org/r/22247
Updated CHANGELOG for 0.19.0.