Merge pull request #142 from messica/dev

Fixed bug when saving discount code uses, added pmpro_check_discount_code filter
Readme and version update.
Fixed bug where level cost would sometimes have incorrect pluralization of months/weeks/etc. (Thanks, Kevin Ackerman)
Now checking the child and parent theme for email_header.html and email_footer.html files to use for emails. The child theme is checked first.
Added pmpro_getfile_before_readfile hook (passes $filename and $mimetype params) in getfile.php
Now making sure there are no JavaScript errors on the checkout page. Use the pmpro_require_javascript_for_checkout filter to disable this.