Add method to get supported authentication methods

Hi, Lejdborg

I'm developing simple SCP client software for OSX with your library, and found we application have no method to get authentication methods of server supports.
Even though we can connenct by trialing password-auth and keyboard-interactive-auth in turn. But the program can't determine what is cause: wrong password or unsupported auth-method. 
So I added it.
And fix litte bug found.
Checking its existance of channel/sftp by accessing property make allocate them if they aren't used.

Best regards.