Add 'important' directive (and possibly other admonitions)

[mvriel]

When I tried to parse some documentation that I had written the parser threw an exception that the important directive was unknown. Is it possible to add this?

See http://docutils.sourceforge.net/docs/ref/rst/directives.html#specific-admonitions for more information on the different admonitions

[doakey3]

I am also having this error with the "attention" admonition.