Shutdown WEBrick server used in installed app flow

Starting and stopping WEBrick only controls whether the event loop is
running, it does not start and stop listening on TCP sockets.
Our WEBrick server is starting to listen when it is initialised,
so we should ensure that we shut it down when we are done with it.

Note that shutdown is idempotent.
Merge pull request #94 from programmiersportgruppe/master

Shutdown WEBrick server used in installed app flow