allow subscribe before entering subscription_loop

it should be possible to subscribe to some channels first,
and then enter subscription loop. I still need to fix callback
handling before entering subscription loop, currently get_reply
will return message instead of invoking callback. See #18
group several tests into subtest
change subscription test

now callbacks set with subscribe are expected to be executed
even outside of the subscription loop. See #18
invoke subscription callbacks from within get_reply

this allows to invoke callbacks for subscriptions outside of the
subscription loop. At the same time, get_reply will still return
the message if you don't want to handle it in callback.
bump version 2.31_01