HTTP && HTTPS transport

These changes add support for HTTPS transport of Kerberos traffic, as per the MS-KKDCPP specification. HTTPS currently uses OpenSSL for cryptography (with future plans for NSS support), and a flag to control which cryptography to use (currently OpenSSL or none) has been added to configure.

A post has also been made to the krbdev mailing list for review of these changes on 2013-08-08.

I am taking over this patch set from Robbie Harwood (thanks Robbie!).