Disabled glog signal handler for mesos tests.

Review: https://reviews.apache.org/r/24780
Updated the upgrades.md for 0.20.0.

Review: https://reviews.apache.org/r/24781
Added documentation for Docker containerizer.

Review: https://reviews.apache.org/r/24775
Disabled glog signal handler for stout tests.

Review: https://reviews.apache.org/r/24783
Disabled glog signal handler for libprocess tests.

Review: https://reviews.apache.org/r/24784
Added CHANGELOG for 0.20.0.
Fixed style issues in CHANGELOG.
Updated configure.ac to 0.21.0.
Added HTTP endpoint changes and deprecations to CHANGELOG for 0.20.0.
Updated the CHANGELOG and upgrades documents for MESOS-1409.
Fixed a bug in docker/docker.cpp that causes unresolved symbols on OSX.

Review: https://reviews.apache.org/r/24825
Fixed master to reject completed frameworks from re-registering.

Review: https://reviews.apache.org/r/20507
Enabled glog signal handler for mesos tests.
Enabled glog signal handler for libprocess tests.
Enabled glog signal handler for stout tests.
Fixed help string for "--attributes" flag.

Review: https://reviews.apache.org/r/24840
Fixed "registered" value in slave's /stats.json.
Report bind parameters on failure.

Review: https://reviews.apache.org/r/24939
MESOS-1208: Add -Wno-unused-local-typedefs if gcc version is >= 4.8

Fixes `MESOS-1208`. The issue is that starting from GCC 4.8, `-Wall` includes `-Wunused-local-typedefs` by default. Since `-Werror` is also present, we end up with an error. I think the better fix would be to update boost, but this fix has less risk.

EDIT: Turns out we were already handling this, but only for GCC 4.8. Modify `configure.ac` to handle it for >= GCC 4.8 instead.

Review: https://reviews.apache.org/r/24816
Allow multiple colons for docker image.

Review: https://reviews.apache.org/r/24916