Add support for zero-downtime restart
vendorise tablecloth
Make application exit when listeners return

Necessary so that the temporary child will exit when reloading.
Set tablecloth.WorkingDir from GOVUK_APP_ROOT

This will allow restarts to pick up the new version of the application
when symlinked directories are updated.  See
http://godoc.org/github.com/alext/tablecloth#pkg-variables for
more details
Merge pull request #8 from alphagov/zero-downtime-restart

Support for zero downtime restarts