pt-BR translation

I came across your project while doing some searches and decided to help by adding a Brazilian Portuguese translation.
However, I've found it difficult to translate the date_last_weekday string, as well as the date_next_weekday. I've explained the problem in comments in the file, but it's basically due to gender variation. I think it's a problem in others Latin based languages, but I'm not sure. I think it would need some research about how others languages vary the gender in this situation to find the best way of solve this issue.