Merge pull request #9 from takikawa/master

Update package dependencies
Implemented first take on a conversion from dc-path to curves
Box and pointer diagram
Improved box and pointer diagram
Box and pointer diagrams now work correctly for all acyclic graphs
Box and pointer diagram now supports shared graphs
Updated docs
Slightly prettier diagrams
Added intersection-points
Renamed save-pict-as-png to save-pict. Now supports jpeg and other bitmap formats as well.
Added what will become tree a little tree drawing lib
Add goals, references and comments to tree.rkt
Added reference to Wetherell and Shannon: Tidy Drawing of Trees
Added support for mutable cons cells
Box and pointer diagrams now allows labels for the cons cells
Greatly improved layout of box and pointer diagrams.
Added more comments
Cleaning
Added example: ploting graphs
Added example of filling area between two graphs