Add initializer so that images get added to the manifest and copied to public/assets in pre-compilation in Rails 4.0.0

Hi, this pull request fixes an issue I've been seeing in Rails 4.0.0.  When pre-compiling assets for deployment, the images in this gem were not showing up in the manifest.  They were also not being copied into public/assets.  This was resulting in 404 errors for the images upon deployment.  I'm assuming that something has changed in Rails 4.0.0 to make this update necessary.  

This approach is being used here: https://github.com/thomas-mcdonald/bootstrap-sass/blob/master/lib/bootstrap-sass/engine.rb