Allow for smooth arrows and custom height function

[cwood]

This allows for two things.

• First is that it will allow smooth scrolling for the up and down arrow keys along with the wheel function.
• Second is this allows us to pass a function to tell the smoothwheel to set the height incase the container is wrong. E.g; having a parallax site where the body,html is 100% of the window even though the doucment.documentElement.scrollHeight is the total height.