verify that handoff works in a mixed-version cluster

This is the verification test for basho/riak#407.

How it works is described in the comments at the top of the test.

The thing most people will probably hate is that when the test fails, it takes it five minutes (the `wait_until` timeout) to do so. Suggestions for improvement there are welcome.

I'd be especially interested in feedback from @joedevivo and/or @seancribbs about whether the version-configuration knobs I've exposed jive with what giddyup will want.