Adding BUTTON check to stopCallback method

I ran into the problem where I had `enter` mapped to open an item in a list, and on the same page had a `button` that you could click. If I tabbed to the button and pressed `enter`, the currently selected item would be opened instead of the button action being executed. I feel like this is a pretty common case, especially if you are implementing keyboard shortcuts on your site. Also, I couldn't think of a valid example where you **would** want mousetrap to handle `enter` on a button.