Improved the checking for libnl3 in configure.ac.

Review: https://reviews.apache.org/r/23145
Refactored the scheduler/executor driver to remove calling 'stop'.

Review: https://reviews.apache.org/r/23142
Make sure we pass CXXFLAGS when building Python eggs.

Python's distuils doesn't look automagically at CXXFLAGS, so we need
to pass it along embedded in CFLAGS. This bug manifested on OS X using
clang because we build libmesos with C++11 but not the Python EGG, and
so the Python EGG can't find certain symbols in libmesos (because
C++11 compiled symbols can have different signatures).

In addition to making sure we pass CXXFLAGS, the CXXFLAGS that are
saved in PYTHON_CXXFLAGS (and the LDFLAGS for that matter) are not
finalized when we do our Python configure checks so some code was
moved around to make sure we do Java and Python at the bottom.
HTTP authentication flag for basic/digest HTTP authentication.

Review: https://reviews.apache.org/r/22222
Minor style cleanups and some additional comments.
Split version where N tokens can be specified.

Updated 'split' function in stout/strings so we can choose how many
tokens we want to retreive.

Review: https://reviews.apache.org/r/22825
Simplified strings::split when 'n' != None.
Added 'bool' return value to Containerizer::launch.
Added composing containerizer and --containerizers flag.
Added a 'Docker' abstraction.
Added a Docker containerizer.
Bug fixes and cleanups in Docker abstraction.
Integrated DockerContainerizer.
Updated an example to use ContainerInfo to run Docker.
Add environment test for Docker.
Added pid() in docker/docker.cpp to get the pid of the container.
Added Docker::ps() "all" option.
Added rm() in docker.hpp/cpp to enable users remove containers.
Modify validate Docker to call 'info' instead.
Added Docker unit test, Docker flag and fix issues found.