Pass Product ID to PAP Sale

Useful when configuring specific products within PAP Campaigns
Merge pull request #1 from jessicaoros/patch-1

Pass Product ID to PAP Sale
Readme update.
Now passing the membership level ID as the "product ID" to Post Affiliates Pro.
Merge branch 'master' of github.com:strangerstudios/pmpro-post-affiliate-pro

Conflicts:
	pmpro-post-affiliate-pro.php