Make nose and coveralls coverage levels match.

In general, there is a difference between what nose reports in its coverage report and what the .coverage file includes (which is what is read by coveralls).  These commits make the two reports the same.

At the high-level, the changes are to:

- not provide coverage for networkx.external
- make sure networkx.testing does not get counted as a test module
- make sure to run doctests when running nosetests from the installation directory

Also, the .travis.yml configuration was updated again, providing much more discussion about what packages are installed in which environments.  Please read those comments carefully, and comment back here.  In particular, we might want to consider pip installing NumPy for 3.3 and pypy.  The downside is that this will make those builds take *much* longer.  Because of the various dependency issues, the current coverage levels are:

    2.6  2.7  3.2  3.3  pypy
    85,  91,  89,  80,  80

which gives us an average coverage level of:  85.0