Added an application log endpoint.

@jmcphers @kevinushey Here's the prototype of what we can use to grab the logs from an application. You'll need to grab and rebuild the lucid vagrant image to test this against, or it should make it to production sometime today or early next week, if you'd prefer to wait.

Currently, this should grab a point-in-time snapshot of an app's logs. It should be feasible to do streaming by hitting the log endpoint we're using suffixed with `?tail=1`, but I can't seem to get that to work without tying up the R session indefinitely. The alternative, of course, would be to poll like we do for task logs every couple of seconds.
Feel free to add commits to this PR if you desire.

Thanks!