Changes for masks API

Made it more consistent with blender convensions
and now it's very familiar to curve's python api.

- Made it spline.points.add(count) wich is more
  efficient in cases lots of points are adding.
  (points.new was removed tho).

- Removed mask_layer.splines.add(), now it's only
  mask_layer.splines.new(), which is how splines
  are adding for curves.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57002 954f8c5b-7b00-dc11-b283-0030488c597c