makes ReceiverConfig a class

which behaves like a dict, to enable reloading the allowed target stuff
adds tests for ReceiverConfig
fix yadtreceiver_tests use proper use ReceiverConfig
implements Receiver.subscribe/unsubscribeTarget
yadtreceiver and tests use allowed_targets

instead of targets to subscribe onConnect
ReceiverConfig.compute_allowed_target use targetnames

instead of full path to match the behaviour of configuration['targets']