This changes the thread coordination of Guard/Listen/interactor
by using Listen in non-blocking mode. This should fix some
thread issues on 1.8.7, at least I cannot reproduce it anymore.
In addition the signal handling was changed: If the Pry interactor
is used, then Ctrl-C is delegated to Pry to exit continuation and
Ctrl-D to exit Guard. Without interactor, Ctrl-C exists Guard and
Ctrl-D is ignored. (Closes #376)