Rework how we talk about moving funds

Now that we have MassPay, let's stop making such a distinction between
U.S. and non-U.S. participants. Let's instead just list the ways to add
and withdraw funds, including PayPal and bitcoin. However, let's also be
honest and straightforward about the limitations of using Gittip outside
the U.S.
Fix bug in the FAQ about being a patron