[api] fix architectures api call descriptions
[webui] port from compass to sprite_factory

compass-rails hasn't seen any commit for 2 months and is not rails 4
ready. and all we need from compass is the sprite functionality, so use
a simpler gem: sprite_factory

The drawback is that the resulting image is in git, so you need to call
bundle exec rake assets:resprite if you add icons
[webui] more preparations for rails 4