Don't allow `allow_unused_http_interactions` option to silence other errors.

For #194.
Refactor RSpec support to before/after hooks.

This is necessary to allow us tell if the example
failed or not.  We can tell from an `after` hook, but
not from an `around` hook, unfortunately.
Ensure RSpec metadata doesn't raise extra unused interaction errors.

For #194.
Ensure unused interactions don't raise extra errors when integrating with cucumber.

For #194.
Add changelog entry.

Fixes #194.
Merge pull request #336 from vcr/issue-194

Improve allow_unused_http_interactions option