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

...and $min->randomPrime($max) as well
$r could be a Math_BigInteger object - you just don't know without further context.

With the two parameter example, `$min->random($min, $max)` could work as could `$max->random->random($min, $max)` or `(new Math_BigInteger())->random($min, $max)`. The calling object can be one of the ones that is passed into random as an option but it doesn't have to be.