Add a static negate() method to BooleanNotNode

[phenaproxima]

This method should take any node and return a BooleanNotNode representing the negated node:

// $node is, say, a method call -- like $foo->baz()
echo BooleanNotNode::negate($node); // !$foo-baz()

I will take a shot at this and probably give it a clearer name, like fromExpression().