faq: reword Coinbase sentence to avoid confusions

"Using Coinbase" is an implementation detail that could lead
people to think they need a Coinbase account for getting the
payout, which is not true because a bitcoin address is the
only requirement (Coinbase is just the USD->BTC conversion
service), as pointed out in #310.

So we reword the sentence to avoid this confusing but still
mention Coinbase as an informative detail about what rates
and fees apply.
faq: add link to "full bitcoin integration" github issue
faq: mention bitcoin also in the withdrawal question
Merge branch 'master' into faq-revision
Trim trailing whitespace
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
Merge pull request #1698 from gittip/faq-revision

FAQ revision