Add a new style check for nullptr

[Frederick Gotham]

I think that we should check through code for:

int *p = 0;
int *p = NULL;

and flag it for style because it should really be:

int *p = nullptr;

[Frederick Gotham]

I just found out today that the following command-line option can be used with g++ to forbid the use of anything other than nullptr:
-Werror=zero-as-null-pointer-constant