Whitespace.
Add is_arborescence(), is_branching().
Generalize is_tree(), is_forest() to handle directed graphs.
Add unit test for #1144.
Clean up docstrings.
Add unit tests.
Major rewrite of module docstring.
Minor docstring update again.
Use len(G) instead of number_of_nodes.
Merge pull request #1145 from chebee7i/trees

Generalize is_tree and is_forest to directed graphs. Also fixes #1144.