Update //third_party/protobuf to version 3.

third_party/protobuf/python/README.txt

-Protocol Buffers - Google's data interchange format
-Copyright 2008 Google Inc.
-
-This directory contains the Python Protocol Buffers runtime library.
-
-Normally, this directory comes as part of the protobuf package, available
-from:
-
-  http://code.google.com/p/protobuf
-
-The complete package includes the C++ source code, which includes the
-Protocol Compiler (protoc).  If you downloaded this package from PyPI
-or some other Python-specific source, you may have received only the
-Python part of the code.  In this case, you will need to obtain the
-Protocol Compiler from some other source before you can use this
-package.
-
-Development Warning
-===================
-
-The Python implementation of Protocol Buffers is not as mature as the C++
-and Java implementations.  It may be more buggy, and it is known to be
-pretty slow at this time.  If you would like to help fix these issues,
-join the Protocol Buffers discussion list and let us know!
-
-Installation
-============
-
-1) Make sure you have Python 2.4 or newer.  If in doubt, run:
-
-     $ python -V
-
-2) If you do not have setuptools installed, note that it will be
-   downloaded and installed automatically as soon as you run setup.py.
-   If you would rather install it manually, you may do so by following
-   the instructions on this page:
-
-     http://peak.telecommunity.com/DevCenter/EasyInstall#installation-instructions
-
-3) Build the C++ code, or install a binary distribution of protoc.  If
-   you install a binary distribution, make sure that it is the same
-   version as this package.  If in doubt, run:
-
-     $ protoc --version
-
-4) Run the tests:
-
-     $ python setup.py test
-
-   If some tests fail, this library may not work correctly on your
-   system.  Continue at your own risk.
-
-   Please note that there is a known problem with some versions of
-   Python on Cygwin which causes the tests to fail after printing the
-   error:  "sem_init: Resource temporarily unavailable".  This appears
-   to be a bug either in Cygwin or in Python:
-     http://www.cygwin.com/ml/cygwin/2005-07/msg01378.html
-   We do not know if or when it might me fixed.  We also do not know
-   how likely it is that this bug will affect users in practice.
-
-5) Install:
-
-     $ python setup.py install
-
-   This step may require superuser privileges.
-   NOTE: To use C++ implementation, you need to install C++ protobuf runtime
-   library of the same version and export the environment variable before this
-   step. See the "C++ Implementation" section below for more details.
-
-Usage
-=====
-
-The complete documentation for Protocol Buffers is available via the
-web at:
-
-  http://code.google.com/apis/protocolbuffers/
-
-C++ Implementation
-==================
-
-WARNING: This is EXPERIMENTAL and only available for CPython platforms.
-
-The C++ implementation for Python messages is built as a Python extension to
-improve the overall protobuf Python performance.
-
-To use the C++ implementation, you need to:
-1) Install the C++ protobuf runtime library, please see instructions in the
-   parent directory.
-2) Export an environment variable:
-
-  $ export PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION=cpp
-
-You need to export this variable before running setup.py script to build and
-install the extension.  You must also set the variable at runtime, otherwise
-the pure-Python implementation will be used. In a future release, we will
-change the default so that C++ implementation is used whenever it is available.
-It is strongly recommended to run `python setup.py test` after setting the
-variable to "cpp", so the tests will be against C++ implemented Python
-messages.
-