Point rendering.

This pull request adds basic point rendering to Helix3D using SharpDX. The PointGeometryModel3D object can now be added to your xaml and and bound to a collection of PointGeometry3D objects. An additional rendering technique, RenderPoints has been added and the shader Points.fx has been compiled into _default.bfx. Currently, point rendering renders points black by default with a fixed size of 4 pixels square.

This is in response to #71.