open-close-endpoint-jkakar

- A new `Client.open_close_usage_event` adds support for the proposed
  open/close endpoint.

@mattmanning @mfine For some reason the tests work locally with
`bundle exec turn -Ilib -Itest test` but not with `rake test`.  I'm
not sure why.  In any case, this combined with the branch in
heroku/vault-usage#150 should make it possible for us to replay
billing data with fewer requests.

Fixes #12.