Validator module

What I've done here is abstract the validation to a module called Validator.  We test only once for each case of a nil field, empty field and missing field.  It's simpler than testing 'well what about THIS field?' for every permutation of required field for every request body.

The call will look like this in the API:
```
Echochamber::Validator.validate([:firstName, :lastName, :email, :password], params)
```

Also renamed the ApiRequest to simply Client.