add warning handler for overflows

[alexsquires]

addressing #29

[bjmorgan]

I've never written custom warning code myself, but I think this looks good.
Does the custom warning code need an else clause that raises the original warning if it does not match either condition, or is that impossible?
Ideally there would be unit tests that test triggering of these two custom warning types.

[alexsquires]

closes #29