RFC: Point of interest data and routing

I'll send this in order to get your comments if this is something that could be mainlined and if yes, which parts would be interesting.

The background for this is that the city of Helsinki is creating a new directory of all services they offer and wants to include routing to/from these services as functionality. Some of the functionality, such as one-to-many routing to places that offer certain service and are closest in time or only routing to services which fulfill certain accessibility criteria, need information about various points of interest to exist in the graph.

The branch includes following changes:
* PoiVertex, which adds information regarding individual points of interest
* PoiLinkEdge, which connects the previous to the street network
* ServiceMapGraphBuilderImpl and ServiceMapAccessibilityGraphBuilderImpl, which are examples how to populate points from JSON-files (Sample data for [unit.json](http://www.hel.fi/palvelukarttaws/rest/v3/unit/) and [accessibility_property.json](http://www.hel.fi/palvelukarttaws/rest/v3/accessibility_property/))
* PoiClassTerminationStrategy, which enables routing to n nearest nodes which have the specified poi category.
* Various changes to tie the new classes together.

The API is extended to include the following fromPlace and toPlace:
* poi:[source]:[id]
* poi:category:[source]:[category_id]

The PoiVertex could also be used in conjunction with analyst. In that case the graph could be loaded with the necessary points once at build-time and they could be used to aggregate accessibility information for specific services/other points of interest.