D.TwoD.Arrow: calculate shaftScale directly instead of searching
Merge branch 'master' into arrows
D.TwoD.Arrowheads: some documentation + warning cleanup
Merge branch 'master' into arrows
D.TwoD.Arrowheads: get rid of spurious Haddock comments
Merge branch 'arrows' -- some general cleanup + minor improvements

Conflicts:
	src/Diagrams/TwoD/Arrowheads.hs
D.TwoD.Offset: bug fix -- need D.Trail import