Added API method in the Router API to save graph to disk (in merge_modules branch)

Makes it possible to push a graph object over the wire and save it on hard disk. Reading the graph then allows OpenTripPlanner to use this graph object again when OpenTripPlanner restarts.

If this pull request is accepted, then pull request https://github.com/openplans/OpenTripPlanner/pull/1177 can be closed without accepting it.