To optimize the picker.

If picker has many rollers,and it has some relationship between the rollers' datasources. When one roller changed value,other rollers should change their datasources and the values on picker.
Such as choosing the state's cities by using picker.

Signed-off-by: Felix <yuanlinyue@gmail.com>
Merge pull request #167 from SuperY/OptimizePicker

To optimize the picker.