clean up imports in Diagrams.Prelude
initial skeleton for arrow support
clean up pragmas, imports, and whitespace in D.TwoD.Arc
Merge branch 'master' into arrows
clean up imports/pragmas in D.TwoD
start work on arcBetween function for drawing arcs between two given points
point-to-point arcs
add diagrams-haddock examples for arcBetween
set up package to copy diagrams-haddock images
add a diagram illustrating the arcBetween code [ci skip]
move Arrow to TwoD.Arrow
first glimmers of arrow support

It compiles!  Still needs a lot of fleshing out.  But at this point
it's essentially just adding features.
arrows: a bit more work; get rid of all undefined's
fix import cycle
arrows: some bug fixes, remove m parameter from ArrowOpts (doesn't compile)
comment out connectPath['] for now
use scaleInvPrim for arrow head and tail

Arrow heads and tails are now just paths instead of arbitrary
diagrams.  This is less expressive but simplifies quite a few things
for now.  We can investigate generalizing later.
add a note about connectPath
Added Arrowheads.hs and new version of Arrow.hs
fixed merge conflicts