Bridge finding algorithm

I converted code developed by Peng Yu <pengyu.ut@gmail.com> and first published as a response to the networkx user mailing list on [May 31, 2012][3] as well as to the [devel mailing][2] list.
[2]: https://groups.google.com/d/topic/networkx-devel/rxgYXEVR3Sk/discussion 
[3]: https://groups.google.com/d/msg/networkx-discuss/5NcjIRUdSKc/Xcd6QTiuxDkJ 

The commit I based this on is here: https://github.com/pengyu/networkx/commit/6e79b612ed0d71ed514b3d8cdf82a3765d8363b6

* I moved the code to `bridges.py` in the `networkx/algorithms/` directory. 
* I converted the provided tests into unittests that run (and pass) with nosetest as stipulated in the [CodeGuidelines][1].
* I added documentation following the guidelines.

[1]: https://networkx.lanl.gov/trac/wiki/CodeGuidelines