Create account levels and tie to payment system

[ephracis]

Add the methods user.free?, user.standard? and user.premium? and tie it into a payment system.

Paypal has a subscription system we can use, but there may be other alternatives we should investigate as well.

The User model should track when the last payment was made, and the amount. Then the different methods should use that info to determine whether the account is on the given level. For example premium? should check if the last payment was less then a month ago and if the amount was more than $100.