Roman century dates

Hi Seth,

this is a small dev to allow roman numerals in centuries expressions, as in XXth century, corresponding to standards here in France [see http://en.wikipedia.org/wiki/Roman_numerals#Modern_non-English-speaking_usage]
For now, everything is controlled by a new datetime.conf parameter.

Bye,

Gautier

![Capture d e cran 2013-01-13 a 22 58 03](https://f.cloud.github.com/assets/314697/63293/8a25eb9c-5dcc-11e2-8ca7-e065221f3439.png)