Add __clone, preps for new clean WedbDriver session

This is the encapsulation-preserving iteration on #69 and #367 on Mink.

Basically, I think cloning a driver should be supported and create a distinct "browser head".

The "real world" of this is that with this change I have step definitions that are multi-persona aware. That is, I can write tests that involve "if brad posts this, kelly can see it", etc. without drastic logout/login sequences.