Added containerizer.proto.

Adds containerizer.proto containing protobuf message definitions for
"Launch", "Update" and "Termination". All of those are used in the
upcoming External Containerizer.  These protos are defined within
their own C++ namespace: "containerizer".

Replaced slave::Containerizer::Termination with the
containerizer::Termination protobuf, gaining a serializable
Termination message.

The protoc results are added towards the Python Egg
(containerizer_pb2.py) and the Java package
(ContainerizerProtos.java).

Review: https://reviews.apache.org/r/20141
Added history to metrics library.

Review: https://reviews.apache.org/r/20015
Fixed public includes.