Add arcCCW and friends.  Fix offset joins.

These variants for arc creation hide the complexity of angles and
orderings between them and instead always gives an arc in a fixed
direction (clockwise "CW" or counterclockwise "CCW") from some start
angle or direction to an end angle or direction.
Merge branch 'master' into arcCCW
Change names.
Simplfy arcCCW and arcCW.
Add dirBetween to Diagrams.Direction.
Hide dir.
Merge pull request #221 from diagrams/arcCCW

Add arcCCW and friends.  Fix offset joins.