Add Tagbar support for reStructuredText via rst2ctags.

I'm not sure what to do for Windows folks.  The script is actually a python file, so it should work under Windows.  I'm just not sure how to detect its presence.  Should we look for `rst2ctags.bat`, `rst2ctags.py`, something else?

It is rather slick to see the section headings though. :-)
![Screen shot 2013-02-16 at 12 05 26 PM](https://f.cloud.github.com/assets/448087/163720/1b1ed35e-785b-11e2-9d35-e3e7025f00a4.png)