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.
* Remove `Singleton` since `Thread.current[key]` is global (to the thread) anyway
* Add spec to assert temporary listeners are thread local
Add use case for temporary listeners
add note about threadsafty [ci skip]
Merge pull request #16 from krisleech/temporary-listeners

Allow subscribing of listener(s) globally for the duration of a block [issue #8]