Fixed a compiler warning using UNREACHABLE in zookeeper.cpp.

Review: https://reviews.apache.org/r/19719
Enhanced DRF by including past allocations.

The DRF allocator (master/drf_sorter.hpp|cpp) currently only uses a
framework's share to determine an ordering for allocating
resources. When there are very few resources to go around it's
possible that a framework will get allocated resources (making it have
a dominant share larger than others), use those resources, then return
those resources (making it's dominant share equal to the share of
others), and then get reallocated the resources _again_ because its
dominant share is equal to others. This can cause starvation for
frameworks.

Review: https://reviews.apache.org/r/19090
Fixed a bug in master to properly reconcile completed frameworks
during slave re-registration.

Review: https://reviews.apache.org/r/19809
Adds Mesosphere to Powered By Mesos list.
Avoided unnecessary retries during log fill by using a higher proposal
number.

Review: https://reviews.apache.org/r/19852
Fixed make distcheck to include repairer.hpp.

Review: https://reviews.apache.org/r/19784
Fixed make dist to properly extract PicoJSON.

Review: https://reviews.apache.org/r/19777
Updated CHANGELOG for 0.18.0-rc6.
Fixed libprocess Makefile to include c++11 headers.

Review: https://reviews.apache.org/r/19878
Updated CHANGELOG for 0.18.0-rc6.
Removed obsolete setting of framework id in executor.

Split from https://reviews.apache.org/r/19795/. This patch removes an
obsolete piece of code in the slave which sets the framework id in the
executor info. It has already been marked for deletion and significantly
simplifies the new reliable reregistration code.

Review: https://reviews.apache.org/r/19877
Moved slave disconnection into new Master::disconnect(Slave*).

Moved code to disconnect a checkpointing slave out of Master::exited()
and into a new disconnect(Slave), to be re-used by slave
authentication code. Also renamed deactivateSlave() to
shutdownSlave(), since it actually sends a ShutdownMessage and calls
removeSlave(). This avoids confusion with deactivateFramework, which
is more analagous to disconnect(Slave).

Review: https://reviews.apache.org/r/19006
Improved Javadoc (use @param, @return, etc).

Review: https://reviews.apache.org/r/19859
Added os::environment.

Review: https://reviews.apache.org/r/19948
Moved subprocess and cleanup method to new cpp.

Review: https://reviews.apache.org/r/19416
Added optional environment map to subprocess.

Review: https://reviews.apache.org/r/19162
Adjusted the includes in stout/net.hpp to avoid definition conflicts caused by netlink.
New and improved stout README.
Updated os::read(fd, size) to not seek.

Review: https://reviews.apache.org/r/19991
Enable automake 1.14.

Review: https://reviews.apache.org/r/19854