Use is_readable() for autoloading

instead of file_exists() as is_readable() also checks file permissions
Merge pull request #2277 from maxrice/patch-2

Use is_readable() for autoloading
spaces
Image optimisation
Hello WooCommerce 2.0 beta 3!
Merge branch 'master' of https://github.com/woothemes/woocommerce
Further optimised pngs using http://pngmini.com/