Fix for svg fonts. Fixes #60

This should fix issue #60. Now the file content is also used to determine the file type images and fonts.
If I merged the last PR with this kind of class overriding, that was a mistake. Please use a util namespace with pure functions such as:

```
module Middleman::Sprockets::Util

  module_function

  def extname?(path, *exts)
    exts.any? { |e| path.extname == e }
  end
end
```