[WIP] Unique validation predicate. 

I've so far I've converted this from https://github.com/hexpm/hex_web/blob/master/lib/hex_web/validation.ex#L29 to the new format. And documented it. 

I'll need to test it, which I assume I will want to have a mocked up repo for. 

Any advice/feedback is welcome. 



Also, let's remove downcase/case_sensitive option here. The best way to handle this is to actually downcase the e-mails (or your field) before entering the database. We could add a note to the README that case_sensitive depends on the database (MySQL is not, PGSQL is) and if you want case_insensitive lookups, you should convert the data prior to entering the DB.