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