Keepalive fixes

- Replaced put statement with debug
- Added include for Loggable
- Reset @unresponded_keepalive_count before raising Net::SSH::Timeout
  - When passing Net::SSH.start a block, the ensure statement calls
    connection.close, if the connection is restored at this point, the close
    call will still fail since the unresponded_keepalive_count > 0
Merge pull request #206 from detiber/fix_keepalive_output

Fix debug output in keepalives