SimpleForm hacks

Globally adds class to collection inputs as requested by @demwunz
Refactor

 - Moves shared examples into own files from Rails helper
 - Delegates Form#column_for_attribute to target object, allowing,
   SimpleForm to reason about column types and infer correct input types
Extend SimpleForm for progressive disclosure

API added to SimpleForm form builder for adding data attributes to
each radio button in a radio button collection
Merge pull request #18 from ministryofjustice/simple-form-hacks

Simple form hacks