make the pytest target more useful
make the test-cov target generate a coverage report in HTML
Merge pull request #2526 from gittip/makefile

Makefile improvements
upgrade VCR
set proper dependencies for the make `env` target
filter fixtures before saving them
remove unnecessary constants
remove unused payday Harness
in tests use participant.id as elsewhere.user_id
move the VCR stuff into its own module
create a Harness.make_balanced_customer() method
support creating a new `balanced_customer_href` in `make_participant()`
initialize balanced only once
update tests
regenerate test fixtures
Merge pull request #2530 from gittip/vcr

Upgrade VCR and optimize our use of it
remove obsolete comment
update stats in a single request at the end of payday