Adapter#start is now non-blocking, Adapter#start! is blocking.

Also, refactor adapters.
Listener#start is non-blocking, Listener#start! is blocking.

Also, deprecate the blocking parameter.
Listen#to is now non-blocking, Listen#to! is blocking.

Also, calling Listen#to! will start the listener right away.
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Merge pull request #98 from guard/start_bang_for_blocking

New Listener#start! method to block the current thread