Complete the management interface

[andymckay-limited-access]

This is the follow on from #85 and covers completing the management interface with the core functionality. Anything not in this core functionality, let's punt to future versions if it becomes too much of a burden:

• add and delete payment methods
• list subscriptions
• cancel subscriptions
• change payment method for a subscription
• list transaction history and show details on a transaction

All bugs should be tagged with "management interface" label.

[kumar303]

is delete account a core feature? It's not on the list but was in #85