Add request duration?

[jqdurham]

We'd like to monitor the performance of our API and hoped that Airborne might allow us to do this natively. It would be nice to have a warning (just a PUTS with the time and warning message) and also the ability to evaluate the request time to trigger a failure.

[bafio]

it should be taken care with #119

[rdalverny]

RestClient incoming 2.1 (still a release candidate) introduces a duration attribute to responses.

This allows to add an expectation on the response time: expect(response.duration).to be < 1.0 for instance. This complements the options introduced by #119 I think.

Two questions:

• it will have to wait at least for the final 2.1 release, but apart from that, would it be ok to upgrade the dependency version right up to 2.1 or is there a requirement to keep an older version? (tests are still green so far)
• I'm not sure about a good expectation name: expect_worst_latency(value)? other?