add a Renderable instance for ScaleInv
new function scaleInvPrim for creating a diagram from a single scale-invariant primitive
further clarify intention of scale-invariant primitives
Merge branch 'master' into scaleinv-prim

Conflicts:
	src/Diagrams/TwoD/Transform.hs
add IsPrim instance for ScaleInv
add IsPrim instances for Path, Ellipsoid, Image, and Text
typo fix in IsPrim instance for ScaleInv
add more comments re: ScaleInv in Diagrams.TwoD.Transform
Merge branch 'master' into scaleinv-prim
Merge pull request #71 from diagrams/scaleinv-prim

New `scaleInvPrim` function