Add jsRoot constructor option

Workers were being constructed with a hardcoded path of
'shapefile.js'. The new jsRoot option can be used to
specify an alternate location. For example:

  shapefile = new Shapefile({
      shp: "testdata/world.shp",
      dbf: "testdata/world.dbf",
      jsRoot: '/js-shapefile-to-geojson/'
  }, function(data) ...