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
Improve sparsegraph6.py docs and include them in TOC
Merge pull request #1029 from gavento/graph6-format-support

Graph6 and Sparse6 formats write support