Add Goldberg-Radzik shortest path algorithm
Fix python2.6 compatibility in goldberg_radzik
Fix python2.6 compatibility in goldberg_radzik
Improve efficiency of goldberg_radzik
Fix bug that goldberg_radzik does not detect negative selfloops
Fix bug that bellman_ford does not detect negative selfloops
Merge bellman_ford and goldberg_radzik tests

A test case for negative selfloop detection is also added.
Avoid printing plain dicts in bellman_ford and goldberg_radzik examples
Split bellman_ford and goldberg_radzik test cases
Fix python2 compatibility
Rename internal function in goldberg_radzik to prevent confusion
Merge pull request #1118 from ysitu/goldberg_radzik

Add Goldberg–Radzik shortest path algorithm