Add argument to stopPropagation method 

Currently returns the following error when closing the sidebar by clicking the 'X' button: "Uncaught TypeError: Cannot read property 'stopPropagation' of undefined"
Merge pull request #2 from bmcbride/patch-1

Add argument to stopPropagation method
README: Added fixed issue #2 to changelog
README: Moved leaflet compatibility note into compatibility section