Added healthy field to TaskUpdate.

As a part of the health check story, this patch adds an optional
healthy field to the task update proto.  The semanticist should be
understood as, if the field is absent - no health check has (yet) been
carried out.  If present, it will either encode unhealthy
(healthy=false) or healthy determined by the HealthCheck protobuf in
the command info proto.  This means that multiple TASK_RUNNING updates
can be sent for a task, updating the state of the last health check.

Review: https://reviews.apache.org/r/21499
Fixed a bug in scheduler driver to properly erase 'savedOffers'.

Review: https://reviews.apache.org/r/22148
Split Authorizer into source and header files.

Review: https://reviews.apache.org/r/22149
Injected Authorizer into Master.

Review: https://reviews.apache.org/r/22150
Changed TaskInfoError and OfferError to Option<Error>.

Review: https://reviews.apache.org/r/22338
Authorized launch tasks.

Review: https://reviews.apache.org/r/22151
Added MockAuthorizer and more authorization tests.

Review: https://reviews.apache.org/r/22190
Authorized resource roles to receive offers from.

Review: https://reviews.apache.org/r/22284
Re-enabled "--acls" flag on master and updated example tests to use authorization.

Review: https://reviews.apache.org/r/22447
Revert "Switched to subprocess.Popen to get stdout streamed from"

This reverts commit 7013f47c3b59314db2b560d67edf1d01afd4a2db.
Updates Mesos version in Getting Started guide to 0.19.0
Simplifies documentation headers.
Added infix operator for IntervalSet.

Review: https://reviews.apache.org/r/22444
Fixed child process leaks in SlaveRecoveryTest.GCExecutor.
Updated verify-reviews to catch exceptions during cleanup.
Initial import for health check
Fixed Protobuf to JSON serialization of optional fields with default values.

Previously, only optional fields which where explicitly set would be serialized regardless whether default values was present.
This patch adds those fields explicitly.

Review: https://reviews.apache.org/r/21246
Cleaned up JSON work
Snapshot of current health check program.
Add task health check