Update Mexican holidays

Some important dates of Mexican holidays were missing, so they were added. Also, changes were made to the file to move some dates to a more appropiate category, i.e. Mother's day was moved from "civil" to "cultural". In Mexico the day off and the actual holiday may be different days, because the day off is moved to monday on some cases; that's why some dates has the word "Descanso" at the beggining. More metadata, notes and sources for the holidays were also added.