Generate proto sources and headers in build.

This reverts commit 96e501eea8ef55e617dc8a7cef986db79b3d0853.
Note: This project will no longer build with the provided Makefile
      as it does not generate the protobuf sources and headers.
Require Python 3.3 and CMake 2.8.8.

Require CMake 2.8.8 for Python 3 support in find_package().
Require Python 3.3 for u'unicode' support which is used in protobuf
generation. This removes nanopb's dependency on Python 2 by converting
the protobuf generation script to Python 3.
Update build instructions in README.md.