+Polylines, +Fitbounds, Center optional

Added `polylines` with options to view helper. http://leafletjs.com/reference.html#polyline
Added `fitbounds` to view helper. http://leafletjs.com/reference.html#latlngbounds
Made `center` optional when creating a map as it can conflict with `fitbounds` and is optional in leaflet.

So now a map() call can look like this:

    map(
      markers: markers,
      polylines: [ { latlngs: [<latlngs>, ... ], options: {color: "green", <any_leaflet_polyline_option>} }, ... ],
      fitbounds: [ <latlngs>, ... ]
    )