add candidate skeleton
add category skeleton
add platform model skeleton
add `votes_count` to platforms table
add PlatformsController#index
eager load `candidate` and `category_instance` of platforms
allow platforms details to be null
add Platform#candidate, #category
refine platform response format
fix sprging bug

- http://stackoverflow.com/questions/23215454/load-error-when-running-rails-console
add doc
Merge branch 'feature/platforms' into develop