add proper version negotiation

[francoismichel]

Until now, the version was strictly checked against the software version.

Now that the project gains in popularity and that a spec is coming, other people may try implementing the protocol.

This adds a version negotiation mechanism that is still quite strict.

• Add the concept of protocol experimental spec version, this currently implements the alpha-00 experimental spec version.
• Do not check the software version during the negotiation, except for versions before alpha-00, where the implementation name then MUST be francoismichel/ssh3
• Right now, the server supports clients with version alpha-00 and older, i.e. clients with implementation name francoismichel/ssh3 and software version between 0.1.4 (included) and 0.1.5 (included)
• Future clients implementing future draft versions will have to adapt their version to the server version. If the server does not support the client version, the client has to retry a new connection with a version that matches the server's version. A message should be displayed to the user, ensuring that the user knows that they should update the server version. Such a message will disappear with stable versions of the protocol.