Introduce circular image styles

A lot of times we want to have circular images. On Google's Material Design guidelines itself, a lot of them have circular images (for contact images, etc.)
http://www.google.com/design/spec/components/menus.html#menus-usage

Rather than using an image editor to crop an image into a circular shape, we can just apply `class="circle"` instead.

Updated the documentation to show how to use the class as well, although I'm not sure if I had put it under the correct section. Update it as necessary.

Thank you.