Add OpenSSL Version Text Method

As I was working on some other features I realized I really wanted to know which OpenSSL cffi was linking against, so I added this basic feature. ```openssl_version_text``` returns the OPENSSL_VERSION_TEXT from opensslv.h.

There was also a small issue with the error msg supplied when an invalid key size is passed to the AES object. If this fix needs to be in a separate PR let me know.