Add validations support

Instance can now validate data for you before it is sent to the server. Validations go in the model declaration of your instance...

```javascript
Comment = new Instance({
    validations: {
        handle: { // name of the attribute/field to validate
            starts_with: { // preset validation rule
                value: "@",
                fail: function() {
                    alert("Your handle must begin with @.");
                }
            }
        }
    }
});
```

A complete list of validation rules can be found on the Instance Wiki.