CHANGES:

• server: be able to stop using a Lwt_switch.t (#52 @dinosaure)
• server: add Pty/Set_env/Start_shell events (#53 @dinosaure)
• client: support password authentication and keyboard-interactive (#51
  @hannesm, reported by @dgjustice #31)
• client: add NIST EC curves (#31 @hannesm)
• client: try public key authenticaion only once (#50 @reynir @hannesm)
• remove (partially implemented) hostbased authentication (#31 @hannesm)
• replace deprecated Cstruct.copy by Cstruct.to_string (#53 @dinosaure)
• remove ppx_cstruct and sexplib dependencies (#54 @hannesm)