Draft cookbook about working with multiple sessions

This cookbook is a draft, based on Behat/Mink#15 and @ctrahey code (https://github.com/Behat/Mink/pull/368#issuecomment-22445924).
In order for this to work with BrowserKitDriver Behat/MinkBrowserKitDriver#21 needs to be merged, I'm not aware if any other drivers need a __clone() implementation.

@ctrahey I'd like you to correct/integrate as you want, I guess we're solving the same issue in our projects.