Add thePointerValue() macro

Allows for pointer comparisons via NSValue. I had need of this while
testing some Objective-C runtime code which makes use of Method and SEL
comparisons.

For example, the snippet

    [[[NSValue valueWithPointer:@selector(sel)] should] equal:[NSValue valueWithPointer:NULL]];

can now be achieved with the much more readable syntax

    [thePointerValue(@selector(sel)) should] equal:thePointerValue(NULL)];

Hope this helps, and open to any suggestions for improvement!