Wait for exec stream copies to complete

[sykesm]

Fixes #11.

[cfdreddbot]

Hey sykesm!

Thanks for submitting this pull request! I'm here to inform the recipients of the pull request that you've already signed the CLA.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this. You can view the current status of your issue at: https://www.pivotaltracker.com/story/show/109385910.

[emalm]

Awesome, thanks, @sykesm! Prioritizing for the Diego team to pull in.

Best,
Eric

[sykesm]

@emalm I'm backfilling the test for the plugin side. I'm not familiar with the cli development process so I'll have to let someone else do it there.

Should be done as soon as I refactor a bit of the InteractiveSession tests.

[emalm]

Ok, sounds good! We probably won't get to this immediately, if you have other changes to make to it.

Man, now I'm bummed out that I hadn't realized to grab the 'emalm' GitHub handle way back when. I'm @ematpl at work and @ericmalm otherwise. :(

[sykesm]

Tests added.