Refactored to class-based, added EDD sanity check

git-svn-id: https://plugins.svn.wordpress.org/easy-digital-downloads-hide-button-prices/trunk@858235 b8457f37-d9ea-0310-8a92-e5e31aec5664
Convert to class based structure

git-svn-id: https://plugins.svn.wordpress.org/easy-digital-downloads-hide-button-prices/trunk@858289 b8457f37-d9ea-0310-8a92-e5e31aec5664