update libssh2 binding to version 1.11.1

- Add new crypto engine types and security key functionality
- Add new error codes and host key types
- Add session timeout and crypto engine methods
- Update type names to match upstream conventions
- Deprecate session_callback_set in favor of session_callback_set2
- Add documentation for new security key functions
- Bump package version to 0.1.9

make `session_handshake` available for `macos` by using `posix` switc…

…h. refs ba0f3/ssh2.nim#19

fix libssh2_scp_recv requires a ptr to Stat. refs: ba0f3/ssh2.nim#16

Merge pull request #13 from pswilde/master

revert breaking change in session_last_error proc

Merge @Techno-Fox's version in #5 to make it compiles on Windows, closes

 #11