[refactor] Use autopep8 to format qdot.py (Use spaces instead of tab).
[refactor] inherit object.
[refactor] re format qdot.py
[Add waring message]  Check the existence of  Graphviz.
Modify shebang line. Use python2  execute this program since python means python3 in some system (like archlinux).
Merge pull request #1 from archaicdust/master

format qdot.py to fit pep8