Adding Support for 0.00 Transactions/Coupons

- See WebSharks/Private#403
- Updating internal documentation.
- Updating routines to support 100% off against specific amounts.
- Updating routines to support rrt="", even when there is no underlying subscription (e.g. when 100% free).
- Updating routines to support rr="0" with a fixed term, where there is no underlying subscription (e.g. when 100% free).
- Updating IPN handler.
- Updating Pro Form generators.
- Updating Pro Form Shortcode handlers.
- Updating JavaScript routines for all Pro Forms.
- Updating Pro Form validators.
- Updating Return Page handlers.
- Updating tracking handlers.
- Updating API Notifications.