Remove subtitle support

Subtitle support should not have been part of the image_caption feature branch, so I'm removing it for now. Will add it into its own feature branch.
Add Image Caption Support

Add support for the {% imgcap %} tag to add captions underneath images.
Code courtesy of [Robert Anderson's blog post][1], which in turn is pieced
together from Ted Kulp's solution.

* Moved img sass code from article > &.post > img to article > img, because
otherwise the {% img %} tag doesn't work on pages (e.g. an About page, etc)
* Added article > .caption-wrapper sass for the captioned images
* Added article > img > &.caption sass to prevent a double-shadow box effect

[1]: http://blog.zerosharp.com/image-captions-for-octopress/
Update README.md with Image Caption instructions

Using the newly added support for the image caption tag require a bit of customization by the user. Instructions for adding the `image_caption_tag.rb` file to plugins, and how to use the `{% imgcap %}` tag itself, have been added.