libssh2 1.6.0

This release includes the following changes:

 o Added CMake build system
 o Added libssh2_userauth_publickey_frommemory()

This release includes the following bugfixes:

 o wait_socket: wrong use of difftime() [1]
 o userauth: Fixed prompt text no longer being copied to the prompts struct
 o mingw build: allow to pass custom CFLAGS
 o Let mansyntax.sh work regardless of where it is called from
 o Init HMAC_CTX before using it
 o direct_tcpip: Fixed channel write
 
This release would not have looked like this without help, code, reports and
advice from friends like these:

  Alexander Lamaison, Daniel Stenberg, David Calavera, Jakob Egger,
  Viktor Szakáts, Will Cosgrove, Joe Turpin
 
        Thanks! (and sorry if I forgot to mention someone)

 [1] = https://github.com/bagder/libssh2/issues/1