Adding patch to the Isochrone web API function to allow using custom routerID

Isochrone web API function now takes account of the routerID parameter, if user wishes to use a routerID other than the default.

(Relevant to issue #703)
(Source of patch idea: Philippe Lacour, https://groups.google.com/forum/#!searchin/opentripplanner-users/graphService.getGraph%28sptRequestA.routerId%29/opentripplanner-users/QD_7-xOEVr0/kHvjPcBbTcUJ)