Add OpenFlow Hello message generator and parser (refs #57).
Add Pio::Hello#to_binary_s (refs #57).
Add new OpenFlow Hello section to README.md (refs #57).
Fix reek warning (refs #57).
Rename Hello#to_binary_s to #to_binary (refs #57).
Merge pull request #66 from trema/feature/of_hello

Feature/of hello