Update tests

Remove PHP 5.2
Add WordPress v4.0.1
Set required version to 5.3.2
Update readme
Merge branch 'master' into issue-599
Check PHP version on activate
Check PHP version on plugins loaded
Bump up required WP version
Move to activation hook
Add a link to Help page
Merge pull request #599 from humanmade/issue-599

Drop support for PHP 5.2