Connected component subgraphs copy

Add option to copy (or not) subgraph data.
Rework connected components algorithms as generators.
Fix Floyd-Warshall when weights have 0 weight. Addresses #976.
Clarify comment in code regarding nan weights.
Remove normalization option to current_flow_closeness

Update docs
Fix failing test
Fix tests that fail due to hash randomization
Add example for sorting components by size
Make `nonedge` default to 0.0. Add more comments.
Clarify description of `weight` parameter.
Allow 'weight' to be callable.
For example, weight=lambda attrs: attrs.get('weight', 0).
Handle nonzero edges via masked arrays.
Also, allow custom multigraph_weight opertor functions.
Revert commits 1bca2d1 and 31b5b70.
Update nose configuration to include coverage.
Do not include coverage from networkx.external
Adjust regex for test discovery.
Now, networkx.testing is not considered a test.
Copy nose configuration to installation directory before testing.
Better comments about TravisCI installations.
Clean up comments a bit more.
Attempt to submit coverage only for 2.7
Merge pull request #993 from chebee7i/coverage

Make nose and coveralls coverage levels match.