Update metrics::Timer::stop to return elapsed time.

Review: https://reviews.apache.org/r/24537
Added batch sizes and timings to the Registrar logging.

Review: https://reviews.apache.org/r/26535
Added markdown formatting to the committer's guide.
Added committer's guide to the documentation index.
Removed unused resources and duplicated attributes from Slave.

Review: https://reviews.apache.org/r/26470
Moved executor checkpointing code from the Executor constructor.

Review: https://reviews.apache.org/r/26525
Added libprocess finalization.

Introduced a finalize() function to match the initialize() function in
process.hpp. Use this at the unit test runner for libprocess to start
getting more valuable valgrind feedback.
Added cleanup of remaining Processes in ~ProcessManager() as a start.

Review: https://reviews.apache.org/r/26533
Changed modules to accept relative path or file name for library name.

Rename "path" in Modules protobuf to "file" since a "path" always
referes to an absolute or a relative path, whereas "file" may refer to a
path or a file name. If "file" contains a slash ("/"), it is considered
an absolute or relative path, otherwise if is considered a file name.
For file name, dlopen() automatically does a standard library path
search to find the absolute path.

Also, return a better error message if an empty module name is provided.

Review: https://reviews.apache.org/r/26529
Exposed p90/95/99 for tcp rtt container stats.

Review: https://reviews.apache.org/r/26396
Made the StatusUpdateManager injectable for mocking purposes.

Review: https://reviews.apache.org/r/26579
Added --module flag for Mesos master.

Review: https://reviews.apache.org/r/26508