Update translations: Welsh (cy), Italian (it), Slovak (sk).

 * UPDATE Italian (it) gettext file.
   Thanks to Francesca Ciceri.
 * ADD Welsh (cy) translations.
   Thanks to huwwaters and littlegreykida.
 * ADD Slovak (sk) translations.
   Thanks to Michelozzo and Svistwarrior273.
Update CHANGELOG with translations updates.
Add Makefile directives for building/cleaning documentation.
Remove unused unittest lib/bridgedb/test/bridgedb.conf file.
Add AUTHORS file.