Add direction flag for roundabout traversal.

(2016-03-22T08:53:58Z) @zerebubuth : originally created valhalla/tyr#108

In the UK we drive on the left, which means we traverse roundabouts clockwise (from above). Valhalla gets this correct in the guidance and route output. It would be helpful if manoeuvres of type `kRoundaboutEnter` and `kRoundaboutExit` indicated whether they were clockwise or anti-clockwise so that the app or website displaying the route could show a left- or right-handed icon to represent the manoeuvre.