Added Pinterest plugin

Hi filaments,

Great job with SocialCount. I am implementing it for a site I'm working on now that also needs a Pinterest Pin It button, so I threw in a quick plugin for it.

I'm not a PHPerson so I didn't add the service for fetching the pin count. Since we aren't displaying counts on this project I haven't been using that. If we do, I may take on porting it to ruby. Did you just frown?

I'm not sure what the proper procedure should be for adding an icon to the image file. I just photoshopped in the new one and added the proper css in the plugin css.

Pinterest Pinning requires two more pieces of data, so my plugin simply expects those as data attributes on the parent socialcount element. Here's an example:

```
<ul class="socialcount" data-url="<%= share_page_url %>" data-media="<%= share_image_url %>" data-description="<%= share_page_description %>">
  <li class="facebook"><a title="Share on Facebook"><span class="social-icon icon-facebook"></span><span class="count">Like</span></a></li>
  <li class="twitter"><a title="Share on Twitter"><span class="social-icon icon-twitter"></span><span class="count">Tweet</span></a></li>
  <li class="pinterest"><a title="Pin It"><span class="social-icon icon-pinterest"></span><span class="count">Pin It</span></a></li>
</ul>
```

Cheers