Make Paperclip.config.root option to be effective

As @mkastner mentioned in #8, dm-paperclip uses `/public` on the system root as base path for saving attachment files by default.

Cause for this was `:path` key in `Paperclip::Attachment#default_options` uses `:rails_root`. If you use this gem without Rails, the interpolation always results in `/public`.

I modified the interpolation to refer `:web_root` instead of `:rails_root` and `Paperclip::Interpolation#web_root` refers `Paperclip.config.root` to allow users to customize base path for attachment files.