Add executable bit.

This allows us to take advantage of the hashbang by running the file directly,
via `./py-ascii-graph.py`, instead of having to specify the python interpreter
explicitly.
Allow importing as a module.

1. Don't do a test print on when the file is imported.
2. Don't use dashes in the filename, since Python modules can only use
   alphanumeric characters and underscores.
Package.

While I haven't uploaded this to PyPI (since it's not mine), I have built and
installed it locally, so I'm reasonably confident it all works.
Add Sphinx documentation-generation files.

I ignored the actual generated files under the assumption you'd use
ReadTheDocs, which generates them itself.
Add invoke tasks.

It's so much nicer than remembering all these different commands, neh?
Alter docstrings to use Sphinx-parseable syntax.
Merge pull request #1 from xiongchiamiov/add-executable-bit

Add executable bit.
Merge pull request #2 from xiongchiamiov/package

Package