Expand billing blueprint to let users subscribe

[nickjj]

The billing blueprint will be modified to do the following:

Update: Striked items are complete and on master.

Membership

• Users can pick a plan from the pricing table
• Users will be able to enter their billing info and apply the membership
• Users can at any time upgrade or downgrade their plan
• Users can cancel their membership
• Users can update their billing info at any time
• Users can view their billing history and upcoming payments
• Coupons can be applied to a subscription

  Credit card

• Save card type, last 4 digits and expiration date for each subscriber
• Track when credit cards are going to expire soon
• Check, mark and remind users of "expiring soon" cards daily

  Coupons

• Mark coupons as invalid once they have expired.

  Util related

• A means to convert cents to dollars and the other way around
• Currency formatting template helpers