Regenerated C files with upgraded cython

Updated manylinux wheel builds. Updated travis cfg. Updated libssh2 s…

…ubmodule upstream

Migrated session userauth_* functions to specific error handling code…

… for exceptions, updated tests.

Migrated handshake and startup functions to error handling code. Added handshake in non-blocking mode test.

Appveyor fixes

Added ssh agent forwarding implementation.

 Bump embedded openssl version

Added error code exception handling to channel. Updated tests for bad…

… use of libssh2 API.

Updated sftp open documentation, example and readme for compatibility.

Resolves ParallelSSH#13.

Updated changelog

Enable embedded libssh2 builds by default.