type=url for mongoose.SchemaTypes.Url

Using [mongoose-types](https://github.com/bnoguchi/mongoose-types) we get Email & Url. It'd be cool is Url was type=url, and didn't make an error like this:

```
/Users/konsumer/Desktop/forms/node_modules/forms-mongoose/index.js:49
throw new Error('Model does not have forms.type, probably on a virtual', p
^
Error: Model does not have forms.type, probably on a virtual
at get_field (/Users/konsumer/Desktop/forms/node_modules/forms-mongoose/index.js:49:11)
at Object.module.exports.create (/Users/konsumer/Desktop/forms/node_modules/forms-mongoose/index.js:109:17)
at Object. (/Users/konsumer/Desktop/forms/app.js:6:18)
at Module._compile (module.js:456:26)
at Object.Module._extensions..js (module.js:474:10)
at Module.load (module.js:356:32)
at Function.Module._load (module.js:312:12)
at Function.Module.runMain (module.js:497:10)
at startup (node.js:119:16)
at node.js:902:3
```