Fixed a conflict with WP Super Cache.

git-svn-id: http://plugins.svn.wordpress.org/delightful-downloads/trunk@847281 b8457f37-d9ea-0310-8a92-e5e31aec5664

Readme spelling.

Fixed button text colors.

Fixed button text colors.

Added hook to download invalid on process-download.php

Fixed fatal error on activation.
Updated version and changelog.