Expanded utilities for working with listeners and callbacks in event driven designs
I added "CallbackHelper" to go along with "ListenerHelper"...ListenerHelper is still the implementation to use with runnable listeners, but CallbackHelper can help reduce code for interfaces where callbacks have multiple functions, or arguments that need to be provided.
Because of this I put both into a new package (org.threadly.concurrent.event).  The old ListenerHelper is now marked as @Deprecated, and will be removed in 3.0.0
Issue #95 covers eventually removing this for the 3.0.0 release

Eventually if this package gets too big, we may seperate this into another project under the org.threadly groupings.  Right now it is small enough that I don't think this is warrented.
Merge pull request #96 from threadly/unstable_staging

Expanded utilities for working with listeners and callbacks in event driven designs.