Rename protobuf Ruby module to google/protobuf and rework its build
system. The Ruby module build now uses an amalgamated distribution of
upb, and successfully builds a Ruby gem called 'google-protobuf' with
module 'google/protobuf'.
Merge pull request #134 from cfallin/master

Update Ruby build setup to produce gems successfully. Include upb as a single .c / .h file.