Async

The operation deferral has now been hand-tested.
Quoting henn (2014-10-17 17:13:57)
>    Does this do anything?

@gsilvis and I talked about this -- he didn't want to rely on the garbage
collector to pick up the session object, which I thought was sensible.
There's certainly a better way to go about this, and I made a few
suggestions out of band, e.g. see if sessions support python's  `with`
statement, put the session creation/teardown outside the api calls
proper...

We're probably not dealing with this at all in other places, though I'd
have to look.