reindent to 2 spaces
remove unused second argument to forEachDir callback
combine recursive and non-recursive methods with an optional parameter that defaults to false
fix CallbackActorSpec tests
fix CallbackRegistrySpec tests
tidy imports
return value for {add/remove}CallbacksFor is unused, just inline the call to modifyCallbackRegistry in the receive method
fix method names, add tests for modifyCallbackRegistry to replace tests for the removed {add/remove}CallbackFor methods