Move PNG assets for rails 4 asset pipeline changes

From http://edgeguides.rubyonrails.org/upgrading_ruby_on_rails.html, 

> In Rails 4.0, precompiling assets no longer automatically copies non-JS/CSS assets from vendor/assets and lib/assets. Rails application and engine developers should put these assets in app/assets or configure config.assets.precompile.

This commit moves the PNG assets from vendor/assets into app/assets, and makes the gem work with the Rails 4 asset pipeline.