Removed some repeating pictures in Atom Feed pages.

Those repeating pictures were due to Clapper's image popup plugin.
- Blog Atom Feed page: Used the idea from ewal.net to strip down the
  relevant `<div>` tags.
- Category Atom Feed pages: It's a bit harder.
    - It took me some time to find out the responsible file:
      `source/_includes/custom/category_feed.xml`.
    - The `{{ page.content }}`, after some passes, still contains some
      Octopress plugin code like `{{% img /path/to/img.png %}}`.  I
      spent some time to write some regular expressions in Ruby to
      change "imgpopup" to "img", so that the popup images will be
      treated like normal Octopress images only in category Atom Feed
      pages.