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.
Now adding classes to the body tag (if the theme supports it through the body_class() function) to aid in design. Classes are pmpro-account, pmpro-billing, pmpro-cancel, pmpro-checkout, pmpro-confirmation, pmpro-invoice.
Fixed members list CSV export for lists with search queries.
Adding $0 orders for free level checkouts (will help reporting).
Fixed multiple instances in ipnhandler.php where the wrong $user_id var was being used.
Added discount code use tracking for PayPal Standard. Saving the use before the user goes to PayPal to pay.
Readme formatting and text update.
Upped version to 1.7.2
Added enddate to daily sales reports to fix graphs.
Upping addon versions.
Minor readme changes.
Sales report fix.
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.