Avoid insertion from front of list in bidirectional_shortest_path.

This applies the patches provided in:
    https://groups.google.com/forum/#!topic/networkx-discuss/ULpDth-GAGU
Increasing allowed time for test on long path.
Removing timed test.

Let's leave unit tests for result correctness and not time complexity.
Merge pull request #1153 from chebee7i/bidirect_patch

Avoid insertion from front of list in bidirectional_shortest_path.