Explicit stubbing methods to aid IDE code completion

I don't know how you feel about this, but adds some explicit methods to Phockito_WhenBuilder (which all just call through to __call()); this means we get some code completion in IDEs (in particular, PHPStorm, which is what I'm using), but perhaps makes the Phockito code a bit messier.

There's also a phpdoc type hint on the global when() function, but hopefully that's not very controversial.

What do you think?