Allow string input on regexp validator

This would allow RegExp validator parse a string instead of requiring the value to be an instance of RegExp. Particularly useful if the schema is transmitted as a JSON object over AJAX for example.

```javascript
// GET /api/schema
{
    error : false,
    code : 200,
    response : {
        id : {
            type : 'Number',
            title : 'ID'
        },
        name : {
            type : 'Text',
            title : 'Name',
            validators : [
                { type : 'regexp', message : 'Not a valid name!', regexp : '^[a-zA-Z]+$' }
            ]
        }
    }
}
```