update matrix demo to use sources_to_targets

[kevinkreiser]

all the matrix end points are special cases of sources_to_targets. we should update the demo to just use sources_to_targets as, its easier to understand and the preferred method from within the service code as well. this will require some ui changes to the demo as we need to some how choose whether we are click points for sources or for targets but otherwise thats about the only change that needs to be made save for a small request format change to accommodate two lists of locations.

this was suggested in valhalla/valhalla#870