Simple(r) validator

@guidow, could you have a look at this?  I think this will add the functionality you're requesting so we can do some forms of validation without using a model object.  Right now `validate_json` supports a callable object but it also supports a [voluptuous.Schema](https://github.com/alecthomas/voluptuous#show-me-an-example) instance too which provides some structure and a good deal of flexibility.  I've experimented with voluptuous on the [agent's side](https://github.com/pyfarm/pyfarm-agent/blob/57d860ea724a5e9fd3c8c4f776ea32e3935e7ae7/pyfarm/agent/http/assign.py#L180) some already and included it here too since it should be useful.