Added External Containerizer.

This patch adds the so-called external containerizer. This
containerizer delegates all containerizer calls directly to
an external containerizer program (which can be specified on
start-up).

The protocol for the interactions with the external program
is as follows:

COMMAND < INPUT-PROTO > RESULT-PROTO

launch < Launch
update < Update
usage < Usage > ResourceStatistics
wait < Wait > Termination
destroy < Destroy

When protocol buffers need to be provided, the Mesos side of
the external containerizer implementation will serialize the
protos on stdin and vice-versa for reading protos on stdout as
drafted in the above scheme.

Review: https://reviews.apache.org/r/17567