Added any.label method

Added any.label() method as described in #414. 

It looked like language.root value was already available in the error message, but it was just not taking precedence over the original key. I changed the order from this:
```javascript
var localized = this.options.language;
this.context.key = this.context.key || localized.root || Language.errors.root;
```

To this:
```javascript
var localized = this.options.language;
this.context.key = localized.root || this.context.key || Language.errors.root;
```

I then added the any.label() method as a convenience method. I believe it should be set up so that language.root takes precedence over any.label().