fix graph() constructor
fix topological sort for undirected graphs
add/fix tests for topological sort
Merge pull request #83 from mlubin/master

fix graph() constructor
Merge pull request #84 from mlubin/topo

fix topological sort for undirected graphs
Update vertex_edge.rst

Add missing word.
Merge pull request #85 from wsliang/master

Update vertex_edge.rst