support $ref

I would love to see support added for the ```$ref``` keyword. I have several error messages that are used throughout my schemas, and it would be helpful to only have to define them once.

```js
// error schema
{
"$id": "errors",
"string": "must be a string",
"basicObject": {
"type": { "$ref": "#/string" }
}
}

// schema
{
"type": "string",
"errorMessage": { "$ref": "errors#/basicObject" }
}
```