Add a method to get the attachment id from the image url
Imported unit tests do not have the correct attachment ids included in the image class. This method removes the `wp-image-` class from the picture, which should clear up problems with imported posts.
Picturefill.js switched to spans also
The `wp-image-` class is no longer required.
Add an example to remove the 20px buffer from media query breakpoints
Add information about caching to the readmes