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!
Merge pull request #2058 from gittip/refactor-vcr

Move VCR setup further up, onto Harness class