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
 o WinCNG: fixed backend breakage
 o OpenSSL: caused by introducing libssh2_hmac_ctx_init
 o userauth.c: fix possible dereferences of a null pointer
 o wincng: Added explicit clear memory feature to WinCNG backend
 o openssl.c: fix possible segfault in case EVP_DigestInit fails
 o wincng: fix return code of libssh2_md5_init()
 o kex: do not ignore failure of libssh2_sha1_init()
 o scp: fix that scp_send may transmit not initialised memory [3]
 o scp.c: improved command length calculation
 o nonblocking examples: fix warning about unused tvdiff on Mac OS X
 o configure: make clear-memory default but WARN if backend unsupported
 o OpenSSL: Enable use of OpenSSL that doesn't have DSA
 o OpenSSL: Use correct no-blowfish #define
 o kex: fix libgcrypt memory leaks of bignum [2]
 o libssh2_channel_open: more detailed error message
 o wincng: fixed memleak in (block) cipher destructor
 
 
This release would not have looked like this without help, code, reports and
advice from friends like these:

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

 [1] = https://github.com/bagder/libssh2/issues/1
 [2] = https://trac.libssh2.org/ticket/168
 [3] = https://trac.libssh2.org/ticket/244