Allow authentication keys to be passed in memory.

All credits go to Joe Turpin, I'm just reaplying and cleaning his patch:
http://www.libssh2.org/mail/libssh2-devel-archive-2012-01/0015.shtml

* Use an unimplemented error for extracting keys from memory with libgcrypt.

docs/libssh2_userauth_publickey_frommemory.3

@@ -0,0 +1,57 @@
+.TH libssh2_userauth_publickey_frommemory 3 "1 Sep 2014" "libssh2 1.5" "libssh2 manual"
+.SH NAME
+libssh2_userauth_publickey_frommemory - authenticate a session with a public key, read from memory
+.SH SYNOPSIS
+#include <libssh2.h>
+
+.nf
+int libssh2_userauth_publickey_frommemory(LIBSSH2_SESSION *session,
+                                           const char *username,
+                                           size_t username_len,
+                                           const char *publickeydata,
+                                           size_t publickeydata_len,
+                                           const char *privatekeydata,
+                                           size_t privatekeydata_len,
+                                           const char *passphrase);
+.SH DESCRIPTION
+This function allows to authenticate a session with a public key read from memory.
+It's only supported when libssh2 is backed by OpenSSL.
+\fIsession\fP - Session instance as returned by
+.BR libssh2_session_init_ex(3)
+
+\fIusername\fP - Remote user name to authenticate as.
+
+\fIusername_len\fP - Length of username.
+
+\fIpublickeydata\fP - Buffer containing the contents of a public key file.
+
+\fIpublickeydata_len\fP - Length of public key data.
+
+\fIprivatekeydata\fP - Buffer containing the contents of a private key file.
+
+\fIprivatekeydata_len\fP - Length of private key data.
+
+\fIpassphrase\fP - Passphrase to use when decoding private key file.
+
+Attempt public key authentication using a PEM encoded private key file stored in memory.
+
+.SH RETURN VALUE
+Return 0 on success or negative on failure.  It returns
+LIBSSH2_ERROR_EAGAIN when it would otherwise block. While
+LIBSSH2_ERROR_EAGAIN is a negative number, it isn't really a failure per se.
+
+.SH ERRORS
+\fILIBSSH2_ERROR_ALLOC\fP -  An internal memory allocation call failed.
+
+\fILIBSSH2_ERROR_SOCKET_SEND\fP - Unable to send data on socket.
+
+\fILIBSSH2_ERROR_SOCKET_TIMEOUT\fP - 
+
+\fILIBSSH2_ERROR_PUBLICKEY_UNVERIFIED\fP - The username/public key
+combination was invalid.
+
+\fILIBSSH2_ERROR_AUTHENTICATION_FAILED\fP - Authentication using the supplied
+public key was not accepted.
+
+.SH SEE ALSO
+.BR libssh2_session_init_ex(3)