title: rails 3.1 asset pipeline: ignore assets from a gem
body: <p>I'm not quite sure what the actual behavior is, so my first question is:<br>
  Are assets (e.g. javascripts) from a gem (in my case Spree) always compiled? I don't use Spree's javascripts, and therefore don't want them to be compiled. I don't require them in my <code>application.js</code> or any other javascript file, but</p>
  
  <pre><code>rake assets:precompile
  </code></pre>
  
  <p>compiles them nonetheless. I just don't want them lying around in my <code>public/assets</code> folder.</p>
  
  <p>So I guess my question is, is there a way to disable compiling javascripts from a gem?</p>