Upgrade RstPreview.py and add a settings file for using docutils which is in other python site-packages(virtualenv)

Nice to meet you.
RstPreview is a nice Sublime Text 2 plugin.

The reason why I sent this pull request is that I usually use virtualenv and don't want to install docutils in global python site-packages.

You can use docutils in other python site-packages when you copy 'RstPreview.sublime-settings' file to 'Packages/Users' directory and write the path of other python site-packages in it.

Because of lack of time, I couldn't rewrite installation explanation in README.rst.

Sincerely yours.