title: paperclip watermark processor unrecognized option `-crop' error
body: <p>Just trying to add watermarks to images via paperclip and imagemagick. Here is gist with my watermark processor:
  <a href="https://gist.github.com/xamenrax/6164246" rel="nofollow">https://gist.github.com/xamenrax/6164246</a></p>
  
  <p>my model is:</p>
  
  <pre><code>  CONVERT_OPTION_PREFIX = "-background transparent -compose Copy -gravity center -extent"
  
    has_attached_file :image,
              :processors =&gt; [:watermark],
              :styles =&gt; {
                :slider =&gt; { geometry: "480x226#", watermark_path: "#{Rails.root}/public/images/watermarks/medium/watermark.png" },
                :big =&gt; { geometry: "670x426#", watermark_path: "#{Rails.root}/public/images/watermarks/big/watermark.png" },
                :medium =&gt; { geometry: "155x160#", watermark_path: "#{Rails.root}/public/images/watermarks/thumb/watermark.png" },
                :thumb =&gt; { geometry: "106x160#", watermark_path: "#{Rails.root}/public/images/watermarks/thumb/watermark.png" },
                :wide_medium =&gt; { geometry: "220x150#", watermark_path: "#{Rails.root}/public/images/watermarks/thumb/watermark.png" },
                :square =&gt; { geometry: "100x102#", watermark_path: "#{Rails.root}/public/images/watermarks/square/watermark.png" }
              },
              convert_options: { slider: "#{CONVERT_OPTION_PREFIX} 480x226", big: "#{CONVERT_OPTION_PREFIX} 670x426", medium: "#{CONVERT_OPTION_PREFIX} 155x160", wide_medium: "#{CONVERT_OPTION_PREFIX} 220x150", square: "#{CONVERT_OPTION_PREFIX} 100x102"  }
  </code></pre>
  
  <p>But when saving image I get error:</p>
  
  <pre><code>Command 'composite -gravity SouthEast '/Users/xamenrax/projects/handmadefood/public/images/watermarks/medium/watermark.png' '/var/folders/26/0fjt3lc101x1714cc5xxkfgc0000gn/T/pro20130806-1681-i05d3l20130806-1681-1yvw0r1' -resize '480x' -crop '480x226+0+127' +repage '/var/folders/26/0fjt3lc101x1714cc5xxkfgc0000gn/T/pro20130806-1681-i05d3l20130806-1681-1yvw0r1'' returned 1. Expected 0
  </code></pre>
  
  <p>What's the problem? Is -crop <code>'480x226+0+127'</code> invalid option?</p>