Gnirehtet v2.0

Changelog since v1.1.1:
 - Add a Rust version of the relay server
 - Handle command-line parsing natively (in Java and Rust)
 - Rename command "gnirehtet rt" to "gnirehtet run"
 - Detect the client version installed on "gnirehtet run" (and reinstall
   if necessary)
 - Optimize TCP checksum computation
 - Improve logs and error messages