added Node, Edge, and Graph classes with method stubs. Fills in __init__ methods, and for Graph nodes(), edges(), and add_node(). Added corresponding unit tests for all aforementioned methods.
implemented del_node() and corresponding unit test.
added unit test for del_edge()
Implemented del_edge() and added EdgeNotInGraphError.
implemented has_node() and corresponding unit tests.
implemented neighbors() and corresponding unit tests
implemented adjacent() and corresponding unit test.
updated readme and added some comments