Allow user-friendly key names in user messages

It would be nice to be able to define a user-friendly key (or "label"?) that would be used when rendering validation messages.  For example:

``` javascript
var schema = {
    firstName: Joi.string().regex(/^[a-z]+$/).required().label('First Name')
};
```

The validation message template would then substitute `{{key}}` with `First Name`  (instead of `firstName`).
