Fixed Zookeeper to recursively create parent paths as necessary.

Review: https://reviews.apache.org/r/11366
Exposed version in "/vars" and "/state.json" endpoints.

Review: https://reviews.apache.org/r/11642
Fixed slave to not send tasks and executor info of a terminated executor,
when re-registering with master.

Review: https://reviews.apache.org/r/11543
Updated the NOTICE to include the correct year, and to fix line
wrapping.

Review: https://reviews.apache.org/r/11696
Fixed slave to properly handle terminated tasks that have pending
updates.

Review: https://reviews.apache.org/r/11694
Fixed master to properly do task reconciliation when slave re-registers.

Review: https://reviews.apache.org/r/11229
Added a new 'statistics.json' endpoint to the ResourceMonitor, this
deprecates the old usage.json endpoint.

This is v2 of the monitoring statistics, added with a new 'statistics.json' endpoint.

The 'usage.json' endpoint is now deprecated, and will be removed at a later point.

From: Ben Mahler <benjamin.mahler@gmail.com>
Review: https://reviews.apache.org/r/11641
Updated 3rd_party licences in the LICENCE file.

Review: https://reviews.apache.org/r/11711
Updated the CHANGELOG for 0.12.0.

Review: https://reviews.apache.org/r/11697
Added a master detector document.
Fixed the name of the master detector document.
Added an Upgrade document.
Updated the CHANGELOG with additional tickets fixed in 0.12.0.
Fixed a typo in the Master Detection filename.
Updated CHANGELOG for 0.13.0.
Updated release tag format in the release script to use the new
0.12.0-rc1 style formatting.

Review: https://reviews.apache.org/r/11786