I wanted to just enable another extension (the graphviz one), but then
I went and did a lot of changes:

- replaced ' with " for consistency with our style guide
- imported new settings (commented out) that current python-sphinx
  (1.0.7) generates when starting a new project; for the keys that are
  different in 0.6 and 1.0+, I left the 0.6 version until we bump our
  documented version
- enabled graphviz; needed for a design doc I'm currently working on
- updated copyright years
- changed list style from single-line to multi-line
- added coverage/ dir to exclude_trees

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>