Added support for validating against a property on the model/object using the length validator

Seeing as this functionality already exists with the numericality validator, I mostly used similar code and added functionality for using properties as values in the length validator. 

The syntax would be the same as it is now with the numericality validator:

```javascript
validations: {
    attribute: {
        length: 'anotherProperty'
    }
}
```

We're already using this in our project, so I figured I might as well do a pull request with it, but I saw you were talking about a rewrite, so it might not be necessary. I'm open to constructive criticism – it is my first PR after all.