Revise protocol buffer binding generation.

Use the CMake "Protobuf" package's protobuf_generate_cpp function, which
we have already been loading but not using in its entirety.

Make use of whitespace more consistent, aping the style used in the
CMake tutorial.
Merge pull request #3 from seh/seh/use-cmake-protobuf-package

Revise protocol buffer binding generation.