base64 encode/decode Bakery cookie values

This is a feature request but also a bug on some systems.

On a fancy nginx infrastructure Bakery's cookies were getting lost after some requests. Lost or just unvalidated, unfortunately the exact fault was never found. But, base64 encoding the cookie value before it was sent and decoding before decrypting has fixed the problem.

Based on PHP comments about mcrypt it seems a good idea that this get into Bakery.

I'll provide this for 6.x-2.x and 7.x-2.x.