tox: remove unnecessary basepython setting

Tox would already use "python" as the default here.
glossary: use proper glossary directive
glossary: fix internal term references

Some terms were shortened to make it easier to reference them.
glossary: rewrap text
glossary: fix casing
glossary: automatically sort entries
glossary: pull out expanded abbreviations

This way you can use both "NaCl" and "Native Client" as terms in the
rest of the documentation.
glossary: add "ZeroVM Shell" for "zvsh"
Merge pull request #11 from mgeisler/glossary-directive

Use sphinx glossary directive for glossary page