nifhell's LDAPS and Credential support in a single pull request.

I take no credit for this work. All credit goes to nifhell (https://github.com/nifhell).

Zakir asked for a single pull request and SSL GC connections. As far as I know, there isn't a separate protocol for SSL GC, just a different port.

Here are the original commits:
https://github.com/nifhell/pyad/commit/f6872140c5d900db80a29c51c15b25712e5059f3
https://github.com/nifhell/pyad/commit/c27937c17e9a14209b4df884a090c1e9c92721c5
https://github.com/nifhell/pyad/commit/b9a6ee41d649a1f7f39230b14980006697d0a041