Ssh

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* server-side SFTP support
* added support for stderr streams on client & server channels
* added a new distinct exception for failed client authentication
when caused by the server rejecting that *type* of auth
* added support for multi-part authentication
* fixed bug where get_username() wasn't working in server mode

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* fixed bug that broke server-mode authentication by private key
* fixed bug where closing a Channel could end up killing the entire
Transport
* actually include demo_windows.py this time (oops!)
* fixed recently-introduced bug in group-exchange key negotiation that
would generate the wrong hash (and therefore fail the initial handshake)
* server-mode subsystem handler is a bit more flexible

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