Updated Graph class API

An update to the Graph classes' functionality, as well as some other miscellaneous things.

New Features

Graphs can now be instantiated from adjacency lists and adjacency matrices.
New Graph.equals method for graph comparison.
New add_edges and add_vertices methods for adding multiple vertices or edges in a single method call.
Pickling functionality.
New add_edge_attributes and add_vertex_attributes methods for adding multiple attributes at a time to a single edge or vertex.
Split the functionality of add_edge into add_edge and set_edge_weight
Change the order of parameters in cg.graph