Support of UUID for model attributes is not functional

Currently if you have a model that has an attribute of type UUID the generator fails to properly parse and build the model.

Output:
```
[design/models.go:33] Unsupported type: 0x6 token in unnamed BuildSource
make: *** [generate] Error 1
```

Example:
```
var UserType = Type("UserPayload", func() {
Attribute("id", Integer, "Unique User ID")
Attribute("real_name", String, "Actual name of user")
Attribute("user_name", String, "Username of user")
Attribute("email", String, "Email address of user")
Attribute("slack_id", String, "Slack ID of user")
Attribute("token", UUID, "Generated UUID of a user")
})
```

```
var UserMedia = MediaType("application/vnd.user+json", func() {
Reference(UserType)

Attributes(func() {
Attribute("id")
Attribute("real_name")
Attribute("user_name")
Attribute("email")
Attribute("slack_id")
Attribute("token")
})

Required("real_name", "user_name", "email", "slack_id")

View("default", func() {
Attribute("id")
Attribute("real_name")
Attribute("user_name")
Attribute("email")
Attribute("slack_id")
})
})
```