Merge pull request #18 from messica/master

Fixed a few bugs.
Now applying end date extension filter to woo commerce checkouts as well. So if an existing member purchases a product for their level that has an end date, their end date will be extended from the old end date. (Thanks, trishacupra)
Fixed the pmprowoo_checkout_level_extend_memberships() filter added in 1.2.5.
Fixed bug with getting the expiration_number for levels with an X months expiration. (Thanks, Arnaud Devic)
Commented out filters on "woocommerce_order_status_pending" and "woocommerce_order_status_processing" hooks. This keeps PMPro from removing a user's membership level when they are renewing which can cause issues. (Thanks, Trisha Cupra and others.)
Fixed bug where startdate was not being set correctly for new users. (Thanks, liferaft) This script can be used to fix startdates for old members: https://gist.github.com/strangerstudios/4604f62e9812cf3afde7