es_ES: Add male and female names data and accessors

This PR adds male and female first names data to the es_ES locale.

I've followed the style in the rest of the file, even though it doesn't really follow the guidelines of the project. I would format it as part of a different PR.

I've taken the 100 most common names for newborns in Spain in 2013 from the National Institute of Statistics:
http://www.ine.es/daco/daco42/nombyapel/nombyapel.htm

I hope this is as helpful to other people as it has been to me.