Move VCR setup further up, onto Harness class

In #2036 we started using VCR to freeze HTTP API calls into test
fixtures. This is great! Let's do more of that! This commit moves the
VCR configuration upstream from where it began (in test_billing.py) into
the gittip.testing.Harness class. Now all tests can make expensive HTTP
API calls with impunity. In fact, this knocks out a XXX comment, because
we had one API call elsewhere in the test suite. VCR transparently took
care of it for us! I was quite pleasantly surprised to find
TestPages.yml. Yay VCR! :-)

My first motivation in doing this, however, was to be able to write a
test for regression I'm seeing with payday (#2057). Onward!
Factor a BalancedHarness out of the test suite

We want to exercise Balanced from two different test scripts. This
commit moves the code common to both into gittip.testing.balanced.
Depends on #2058.
Refactor the tests for charge_on_balanced

Picking up from #2059, this commit refactors the existing tests for
charge_on_balanced (which is where the bug in #2057 lives) to use VCR
instead of mock objects. I was hoping it would make the tests cleaner
but it looks like I don't quite fully understand how to use VCR yet. :-/
Here's two failing tests for some payday bugs

These address regressions introduced by #2036.
Handle the Balanced API more saferly during payday

This makes us more robust in the face of failures communicating with the
Balanced API. Now we've got a blanket try/except around the parts where
we talk to Balanced, so we will never totally crash because of a
mishandling of some sort. The ach_credit side is not as well-factored
as the charge_on_balanced side, and is not tested as a result. This
commit nonetheless attempts to implement the same error handling on both
sides.
Take out probably misguided check for merchants

At https://github.com/gittip/www.gittip.com/pull/2036/files#r9757101
@mjallday suggests that we don't need to check that merchants are
underwritten, as that constraint now only applies to those receiving >
$100,000/yr, and we don't have anyone there yet. Moreover, I believe
this condition is backwards, and would result in no-one getting paid!
Without any kind of condition here, I suspect that we'll fill our logs
with NoResultError()s for everyone who is due a payout and has a CC on
file with Balanced but not a bank account. Better that than not doing
any payouts. :-)
Merge pull request #2058 from gittip/refactor-vcr

Move VCR setup further up, onto Harness class
Merge pull request #2059 from gittip/balanced-harness

BalancedHarness
Merge pull request #2063 from gittip/refactor-charge_on_balanced-tests

Refactor charge_on_balanced tests
Merge pull request #2064 from gittip/fix-payday-regressions

Fix payday regressions
Merge branch 'master' into rename-balanced-column