Add find_cycle()
Add example for find_cycle.
Docstring updates.
Minor docstring edit.
Update docstrings and unit tests.
Correct doctest in find_cycle.
Move function level import to module level.
Use set literal instead of set().
PEP 8.
Raise NetworkXNoCycle when no cycles are found.
Merge pull request #1194 from chebee7i/findcycle

Add function to find a single cycle.