adding an explicit background size for the wrapper div

This will allow the map hilights to scale appropriately.  This is especially helpful if you want to use a hidpi version of your map (pixel doubled)

I know that a similar approach was included in this open pull request: https://github.com/kemayo/maphilight/pull/36

However, I think this one is more generally compatible and *only* contains the background sizing modification.
Merge pull request #3 from zbtirrell/respect-size

adding an explicit background size for the wrapper div