Remove check for theme e-mail templates on email

Remove the check for e-mail templates located in themes to ensure the
make_clickable function can run on all e-mails.
Fixed warning when trying to load the "free" gateway on free level checkouts.
Changed all $wpdb->escape() calls to esc_sql() to fix notice.
readme update
Fix missing label end tag and input close

These changes fix an issue where the Membership Levels page will not
appear in IE (tested on IE9).
Merge remote-tracking branch 'upstream/dev' into dev
Update checkout_express.html
removing $
Merge pull request #58 from scottsousa/dev

Remove check for theme e-mail templates on email
Fixed bug where links in the email_header.html and email_footer.html templates were not being converted to true links.
Readme
Fixed loading of local (in the paid-memberships-pro/languages directory) translation files.
Version change to 1.7.2.1