Updated sftp open documentation, example and readme for compatibility. ResolvesParallelSSH#13.

Updated changelog 

Enable embedded libssh2 builds by default. 

Known host API implementation, tests and documentation. 

Added OSX wheel builds for all OSX versions on Travis-CI. 

 Updated manylinux wheel builds Updated osx wheel build

Update openssl wheel version 

Direct tcpip test 

Disable system package builds temporarily 

Update travis OSX build.