add define-vertex-property, define-graph-property

- convert graph-fns-basic.rt
add graphviz tests
add get-edges
graphviz.rkt minor formatting
add graphviz.rkt to run-all-tests
add function option to vertex-property #:init
convert minspan, coloring, and ss-shortpath to use vertex-prop
vertex-property init uses syntax-parameter $v
add define-edge-property
vertex-property fail now uses #:default keyword
convert to use edge/vertex properties: ap-shortestpath, maxflow, boost examples
expose graph properties from library
add docs for graph properties
add get-neighbors
add ->hash fn for graph properties