Add support for Image Captions

[Rober Anderson](http://blog.zerosharp.com/image-captions-for-octopress/) wrote an excellent blog post about implementing Image Captions in Octopress. This commit:

* adds the sass support needed to get image captions to work under Oscailte
* adds an entry to the README discussing how to add the image_caption_tag.rb file to plugins
* also fixes an issue that kept both the img and imgcap tags from working on Pages