Allow pay-ins from PayPal

[pjf]

This came up in http://gittip.freshdesk.com/helpdesk/tickets/68 , and this seems like an obvious feature.

It's quite possibly already in our issues/feature queue, but I can't seem to spot it right now.

[pjf]

Although I imagine we want to limit how much money is sitting in PayPal, given they have a long reputation for freezing funds. But since we do so many PayPal pay-outs anyway, I imagine any funds in PayPal would empty out pretty quickly as part of the payday run.

[chadwhitacre]

Closing as dupe of #63. The conclusion there was that PayPal is highly unlikely to underwrite our use case. We're using them for payouts as a bandaid over #126. We already support payins outside the US so not as much pressure there.

We would need some assurance from PayPal before fully embracing it for Gittip.