3.0 - Create the select widget for the new FormHelper

As part of the overarching FormHelper work in #2267, this set of changes builds the first 'widget'. The widgets will replace the vast swaths of code inside FormHelper with a more modular, extensible and overridable system of widgets. Each input type will be encapsulated in a widget that is responsible for generating the various input elements a form widget requires. The label, wrapping div, and validation messages are separate concerns that the FormHelper will orchestrate together when someone calls a method like `$this->Form->input('Article.user_id');`

I started with select boxes as they are the foundation for other complex types, and are complex enough to get a feel for how the widget system would work. After this is ready I want to tackle radio buttons and date time inputs.