Fixed formatting in getting-started document.
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.
Fixed bug with generated headers.
Added braces to show locked section in Metric.