Graph6 and Sparse6 formats write support

I frequently find myself working with many graphs in graph6 or sparse6 format in my research (mostly because of nauty and their compactness) and I would appreciate if NX supported writing them as well. NX already had reading support for both. I added writing support similar to that of other formats.

I decided to rename some module-local utils of sparsegraph6.py when adding inverse functions to better describe what they do. The unit tests cover all the new methods and are checked against sagemath. The code is new, all should be according to the description at http://cs.anu.edu.au/~bdm/data/formats.txt.

I will write the relevant documentation (mostly already contained in the docstrings) if you decide to pull my changes. I am of course open to suggestions and comments.

Best,
  Tomas