Fix: Use of undefined constant php - assumed 'php'
Fix: 'PHP Notice:  Undefined index:' warnings
Fix up textdomain
Fix heading markdown in README.md
Whitespace
Make 'occurs' options available for translation
Add new display opt-in & opt-in label fields

And display and save the opt-in field's value on checkout.
Only subscriber if opted-in on checkout

Or no opt-in on checkout was required when the order was placed
Whitespace
Set version to 1.1 & udpate changelog
README.md tweaks
Merge pull request #1 from thenbrent/master

Add optional "opt-in" field on checkout