Enhancements to translatable field transforms

The default transform of fields done in `Translatable_Transformation->baseTransform` is suitable for the majority of fields (and as a default). It works well for field types like `TextField`, `TextareaField` and `HTMLEditorField`, but quite poorly for some others, the most obvious being `CheckboxField`.

These changes make it easier to modify how `Translatable_Transformation` transforms fields based on the field type, and implements a new transform for `CheckboxField` as a starting point.

An example of how `CheckboxFields` display on a translated page before and after application of this patch is attached.

![](http://cl.ly/N0dk/Image%202013-02-20%20at%201.22.17%20PM.png)
![](http://cl.ly/N1Ka/Image%202013-02-20%20at%201.02.55%20PM.png)