Connected component subgraphs copy

Add option to copy (or not) subgraph data.
Rework connected components algorithms as generators.
Fix bug in gml writer

Remove unneeded extra recursive call to listify.
It added an extra set of double quotes.
Merge pull request #981 from hagberg/gml-nested

Fix bug in gml writer
Fix Floyd-Warshall when weights have 0 weight. Addresses #976.
Clarify comment in code regarding nan weights.
Graph initialisation support for tuple edge-lists
Merge pull request #984 from ninjin/tuple-init

Graph Initialisation Support for Tuple Edge-lists
Make coveralls work again.
Comments in configuration model about multi-edges

Add comments about scaling of the number of parallel edges and self loops.
PEP-8 modifications
modified duplicate line to use intended graph H.
JSON graph fix to handle unicode dict keys
Make make_str() always return unicode, no matter the Python version.
Merge pull request #989 from hagberg/json-str

JSON graph fix to handle unicode dict keys
Merge pull request #987 from hagberg/986-configuration

Comments in configuration model about multi-edges
Fixes #986
Merge pull request #988 from sbromberger/master

Align current_flow_closeness.py with PEP-8 and remove some duplicate code
Addresses #983
Merge pull request #990 from chebee7i/hagberg/json-str

Make make_str() always return unicode, no matter the Python version.
Simplify make_str() for Py3.
Merge remote-tracking branch 'upstream/master' into hagberg/json-str
Remove normalization option to current_flow_closeness

Update docs