Remove uneeded `big_enough` variable
Wrap `getimagesize` in a conditional staement to check for `allow_url_fopen`
Try again to get the image width and height if `allow_url_fopen` is not available.
re: issue #18
Make the `$content_type` attribute required in the `cache_picturefill_output` method.
Update documentation with ACF example
Update version number
Add link to ACF documentation
Add an index to the examples section
Fix heading hierarchy
Include note about errors with HTML5 elements
Add the style attribute to picturefill.js
re: issue #19
Fix indentation
Don't even try if the `content_type` is not set.