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

* Adds libssh2_userauth_publickey_frommemory macro for libssh2_userauth_publickey_frommemory_ex.