Fix markup.
IDirectedGraph now inherits directly from Container, Iterable and Sized.
Add framework for building Sphinx docs.
Specific documentation improvements.
Make vertex_set and vertex_dict classmethods.
Refactor: move graph creation functions into refcycle.creators; move gc utilities to refcycle.gc_utils.
Merge from master.
Separate docs for the two main classes and the utility functions.
Make Sphinx read the version from the source.
Use correct package directory.
Rename: release-level -> prerelease.
Merge pull request #18 from mdickinson/add-documentation

Add documentation
Fix markup.
Add pointer to Read the Docs documentation from README.
Merge remote-tracking branch 'origin/master' into scc-graph