Prepare graph6 utils for generating/writing

* Rename graph6<->data<->n functions to descriptive names (at all
  occurences)
* Add the other conversion direction
* Add unit tests
Implemented writing graph6 format, plus unit tests
Implemented writing sparse6 format, unit tests
Add credit
Whitespace/tabs and python3ism fixes
Line length formatting to 80 cols
Formatting, add author, whitespace
Whitespace cleanup
fast euler cycle algorithm implemented
documentation updated
compatibility with python3
added reference
Improve sparsegraph6.py docs and include them in TOC
Merge pull request #1029 from gavento/graph6-format-support

Graph6 and Sparse6 formats write support
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