disqus_identifier support, and comment counting

First, thanks for building this!  Here are some changes I've made (sorry to bundle them all in one pull request).

- Allow passing of a `disqus_identifier` to the `Discuss::comments()` method.  This tells Disqus to use the identifier instead of the URL when displaying the comment thread.  Useful if you ever change the URL for a post, which will lose all the old comments, I believe.
- Added `Discuss::counts()` method, which generates Javascript to allow display of comment counts for a post.
- Moved HTML to views instead of inline in the methods.
- tightened up the Javascript code a bit.
- Updated readme with updates.

Thanks in advance.  Hopefully you can incorporate these changes into the package.