Refactored Net_SSH2::$identifier and added unit tests

Currently there's no way to set the SSH-Identifier, this pull request tries to give you the posibility to do this. Now you can use `class MySSH extend Net_SSH2 { function _generate_identifier() { return 'MySSH'; } } ` to overwrite this var

 This new way of generating the identifier made it testable, so I added some unit tests.