Merge cleanups and unit tests from 4.0 branch

[dmethvin]

If none of these are breaking changes, stay with 3.x version. See #142.

[brandonaaron]

In the 4.0.x branch I swapped the delta signs to match the newer wheel event (https://github.com/jquery/jquery-mousewheel/tree/4.0.x#older-versions). Off the top of my head that and not passing the delta values to the handler (they are only attached as properties of the event) are the two breaking changes.

[mgol]

If we're swapping the delta signs, maybe we should also get the event renamed to wheel to reduce confusion? It would also make it easier for external code to work with both old & new versions of the plugin if needed.