tests/server: no nested classes anymore
tests/server: make the server thread accessible

The server thread is now accessible from outside of the class. Now the
user of the server could join on this thread and block until the server
finished.
tests/server: accept an optional host to listen on
tests/server: drop six.string_types dependency
tests/server: renamed all_handler to _handler
tests/server: extract _send_404 method
tests/server: remove try catch, checking existence first
tests/server: extract _send_reponse method
tests/server: defaults are now in a dict