Add topological sort
Fix node/edge attributes not being added in addGraph()
Fix infinite loop on nonexistent attributes in predicates

Attempting to get a nonexistent attribute in a predicate would cause
an infinite loop due to Snap! handling undefined as a return value
specially.
Add edge coloring and custom labels

Also changes the default node fill to white.
Add critical path algorithm example
Merge pull request #21 from Muon/master

Toposort and critical path example