Simplified raycast API

* Added World.rayCast function, taking a RayCast object and the start
  and end points for the ray.

* Removed RayCast.point1, RayCast.point2, RayCast.world, RayCast.repeat
  and RayCast.running. All these properties can be trivially implemented
  on the QML side.

* Renamed RayCast.result to RayCast.maxFraction

* Renamed RayCast.onCast to RayCast.onFixtureReported

* Adjusted raycast example accordingly
Merge pull request #57 from bjorn/simplify-raycast

Simplified raycast API