Allow string IDs

* Allow string input for the id on all methods.
* Add src and dst to edge attributes.
Add remove_node() and remove_edge()
Add documentation.
Ignore pycache and .DS_Store
Return a more detailed dict in get_nodes()
Add py.test unit tests.
Add test info to README
Whoops, forgot to add fixture.
Hide edges too.
Improve test coverage
Change __init__.py to reduce import statement.
Add id_ to node attributes.
Merge pull request #4 from dead10ck/master

String IDs, remove methods, docs, unit tests.