pngs with empty alpha channels can have conversion attempted the first pass

git-svn-id: https://plugins.svn.wordpress.org/ewww-image-optimizer/trunk@856038 b8457f37-d9ea-0310-8a92-e5e31aec5664
remove deleted images from the ewwwio_images table

git-svn-id: https://plugins.svn.wordpress.org/ewww-image-optimizer/trunk@856052 b8457f37-d9ea-0310-8a92-e5e31aec5664
readme updates, version updates to 1.8.0

git-svn-id: https://plugins.svn.wordpress.org/ewww-image-optimizer/trunk@856923 b8457f37-d9ea-0310-8a92-e5e31aec5664
fix issue with undefined function

git-svn-id: https://plugins.svn.wordpress.org/ewww-image-optimizer/trunk@857985 b8457f37-d9ea-0310-8a92-e5e31aec5664