Move assets to app/assets for Rails 4

In Rails 4 "loose" assets (images, fonts, etc.) in `lib` and `vendor`
are no longer precompiled by default. Rails Engines should put their
assets in their own app/assets.
See: https://github.com/rails/rails/pull/7968
Merge pull request #28 from stevenharman/rails_4_compatibility

Move assets to app/assets for Rails 4