Add bin/user_agent_parser script.

The script reads each line from STDIN as a user agent, parses it and
prints back the parsed result.
Handle invalid version.
Inject user agent to avoid multiple initializations.
Document formatters in usage information.
Allow binary to be symlinked.
Merge pull request #16 from rejeep/user-agent-parser-bin

Add bin/user_agent_parser script.