BigInteger: make it so you can do $min->random($max)

...and $min->randomPrime($max) as well
> Might want to support the other way around as well.

I'm not sure what you mean? `$max->random($min)`?

> I am not sure this change is a good idea.

Why not?

> $min->random($min, $max) or $max->random($min, $max) isn't very verbose.

I like `$min->random($max)` better myself but that's not the way Math_BigInteger currently works. Right now you have to do `$whatever->random($min, $max)` and I just don't like that. I don't think it's as intuitive.