Add a new executable bin/protoc-gen-ruby, which behaves as a Ruby code
generator plugin of protoc.

Plugin is the preferred way of adding a support of non-official
language to proto compiler.

You can generate Ruby sources by commands like the following when
this plugin is installed in a location on the path.

% protoc --ruby_out=/path/to/output/dir foo.proto
Merge pull request #10 from yugui/feature/plugin-bin

Add a new executable bin/protoc-gen-ruby, which behaves as a Ruby code generator plugin of protoc.