Containers are balanced on image pull
Containers are sortable by creation datetime (ascending)
interactions.destroy is tolerant of api errors

docker appears to have a lot of reasons to explode when asked to stop and
destroy a container, despite usually succeeding to do as asked despite the
error
Pull logging improvements

To cut down on pull-related log spam:
- Logging only happens when a new image is pulled
- The 'this could take a while' message only takes place at start-up
  (the pull is happening asynchronously, so the log will reflect that
  a new image *was* pulled after the download succeeds)
Selenium wait time after starting an instance bumped to 120 seconds

Apparently, servers with high load (like, say, running 20 simultaneous firefoxen)
may take more than 30 seconds to start selenium.
Remove old images when a pull from a tag tracks a new id
Minor lint/comment