Refactored reading credentials out of master.cpp.

Review: https://reviews.apache.org/r/20301
Added a message string to the slave ShutdownMessage.

Review: https://reviews.apache.org/r/20302
Updated mesos-style.py with already-passing rules.

Review: https://reviews.apache.org/r/20649
Correct and simplify the ProcTests for threads.

Using clone() for creating threads is not advised because glibc expects
thread local storage to be set up in particular ways.

Review: https://reviews.apache.org/r/20632
Add a new boolean CommandInfo.URI.extract.

This is a patch that includes a new `extract` boolean value on
CommandInfo.URI to toggle the auto-extraction of compressed archives
when shipping URIs with an executor or task.

It's worth noting that this changes the behaviour of the
MESOS_EXECUTOR_URI environment variable used. I've added some comments
which outlines the expected format, instead of (e.g)
hdfs://foobar/testing.tar.gz+0 (1 signifying it's not executable)
there is now an extra flag on the end (X or N) defining whether to
auto-extract or not, resulting in hdfs://foobar/testing.tar.gz+0X by
default.

Review: https://reviews.apache.org/r/20674
Added authentication support for slaves.

Review: https://reviews.apache.org/r/18381