Replace os.sep with "/" for js script path

A user reported that leafelt maps generated by this plugin doesn't work on firefox. The cause of this was that if the index.html was generated on a windows machine, it has a separator '\' instead of '/'. Firefox fails to load this JS file since it can't interpret the path. While using os.sep is correct for system paths, for javascript paths, using forward slash would be better.
Merge pull request #104 from spatialthoughts/patch-1

Replace os.sep with "/" for js script path