Append '/' to base url when necessary

Fixes #248.
Is there ever a time when ``baseUrl`` would be an empty string?  Like if you were serving out of a directory where you store the tiles or something?  I only mention it because I ran into this in some of Tangelo's internal URL-handling logic.  I have a special code path there that checks for ``""`` and replaces it with ``"/"``, to avoid causing an error on Tangelo's equivalent of this line.