BUGFIX in tpt.py which resulted in repeated paths in find_top_paths whenever the 'sinks' had more than one entry. Also  fixed tests/test_tpt.py to actually test whether the paths are correct. We need to add a test for multiple sinks/sources as well to avoid these types of bugs in the future.
Changed Flux to flux in tpt._backtrack
Added a test for multiple states/sinks in FindPaths.
Merge pull request #178 from schwancr/fix_tpt

Fix tpt bug where the paths were not output correctly due to issues in tpt._backtrack