Added translations and documentation about the e-mail variables

Several variables (%(threadtitle)s %(threadurl)s, %(boardtitle)s, %(mailsignature)s %(salutation)s and, when appropriate, %(commenturl)s) can be used in the generated e-mails. I have documented them on the configuration page.

I have also added basque (eu) and spanish (es) translations and a po file generating script.