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
Fixed small typo in master module flags description.

Review: https://reviews.apache.org/r/26588
Perform read right after subprocess for docker ps.

Perform read right after subprocess for docker ps.

Review: https://reviews.apache.org/r/26487
Change passing io output future from by reference to by value
Update the Mesos Style Guide with C++11 and naming notes.

Explicitly prohibit the use of namespace aliases. Give examples of
preferable way of using auto.

Review: https://reviews.apache.org/r/25622
Enabled whitespace/semicolon rule for cpplint.

You should always have a space after a semicolon except for few corner
cases

You shouldn't have a space before a semicolon at the end of the line.
There's a special case for "for" since the style guide allows space
before the semicolon there.

Review: https://reviews.apache.org/r/19965
NOLINT a few operator overloads, to enable whitespace/operators

Review: https://reviews.apache.org/r/19967
Enabled whitespace/operators rule for cpplint.

Enabled whitespace/operators rule for cpplint, which covers the following:
You should always have whitespace around binary operators.
Missing spaces around <, >, or =
We allow no-spaces around << when used like this: 10<<20, but
not otherwise (particularly, not when used as streams)
There shouldn't be space around unary operators

Review: https://reviews.apache.org/r/19967
Modify configure.ac to fix --with-sasl.

Fixes MESOS-1839.

Review: https://reviews.apache.org/r/26133
stout: Always use stout ABORT() rather than system abort()

This makes it so any time there is an abort, we get a line number and at least
a basic message as to why there was an abort. If you want a clean(er) exit, use
<stout/exit>.

Also adds an overload which takes a standard string and unwraps it to a
const char * automatically, since a lot of the time we are building strings
to pass them to abort.

Review: https://reviews.apache.org/r/26472