Added Validators to cuttlefish

* Added a validator module and {validator, "name", "description", fun/1) to the schema format
* switched from a lists:partition to lists:foldl since there are now > 2 types of schema items
* also accepted that {Translations, Mappings, Validators} will mostly be thrown around together. Mostly...

riak pull request to test this, make sure you point rebar to this branch
https://github.com/basho/riak/pull/377