Wrap long labels

Wrap labels if they exceed the dimensions of the circle they are labelling.
Fixes #7 where labels were exceeding dimensions of the svg
Center labels in disjoint region

Rather than put the label in the middle of the circle, lets put the label in
the middle of the part of the circle that is disjoint from all the other ones.
Makes diagrams more legible, and helps out with issue #18
minor fix
Merge branch 'master' into gh-pages