Allowing schemas to be passed uninitialized schemas

This allows:

````javascript
var blogSchema = schemata({
    comments:
      { type: schemata.Array(createCommentSchema)
    }
  })
  return blogSchema
````

Instead of:

````javascript

  var blogSchema = schemata({
    comments:
      { type: schemata.Array(createCommentSchema())
    }
  })
  return blogSchema
````