Allow subscribing of listener(s) for the duration of a block [8]
Fix spec in MRI 1.9.2, passing a spec double, which does not respond to `to_a`, to `Array()` causes unexpected message error.
* Allow temporary listeners to be specified as arguments instead of an array
* Add method to Wisper for adding temporary listeners
* Update README
remove `publisher_class` method which is already in spec_helper. It appeared from the future after rewriting history using rebase.