Don't sort nodes, style updates

Nodes might not be sortable so use specified order or list(G).
Small style fixes.
Test DiGraph error.
Split sparsegraph6 into sparse6 and graph6
Drop *list functions in graph6

Use read_graph6 instead of read_graph6_list
Use stringIO instead of temp files
Remove *list functions
Use StrinIO instead of tempfile
Add docuementation, formatting
Formatting, whitespace cleanup
Return graph type if no parallel edges
Add nodes argument
Add docs
Fix StringIO to work with python2/3
Typos, spell check
Add back linefeed on write
Use multigraph in test
Merge pull request #1033 from hagberg/graphsparse6-split

Split graph6 and sparse6, deprecate *list functions