v0.0.4

This release covers 62 commits, and contains many improvements and bug fixes:

• Added a TransformMixin, which provides many convenient methods for transforming graphs.
• Graph.pairs and Graph.edge_weights now support a directed kwarg.
• Added a Graph.subgraph method, for extracting a subgraph as a separate Graph object.
• Added a Graph.remove_edges method.
• Graph.from_edge_pairs now supports the case where weights are provided for symmetric pairs.
• Added a benchmark for neighbor graph construction.
• Added an example script which demonstrates nine methods for building a graph on the swiss roll toy dataset.
• Improved docstrings for methods overridden by Graph subclasses.
• Refactored the Cython code for easier distribution and use.

... and much more! The API is still evolving, but most of the changes should be backwards-compatible with v0.0.3.