Updated and configured SimpleCov for test coverage.

Integration test coverage, anyway.
Finished Payment specs that didn't assert anything.

Also:

  * stubbed a lot of stuff so those tests could pass
  * fixed some other test setup, same reason
  * replaced a nonsensical "don't cancel" test
  * fixed bug where cancel says it worked if Authorize.net
    says it failed but declines to explain why
Removed extraneous `is_active?` method.

`active?` is built in and more idiomatic.
Test refactor: common context for subscription builders.