Settle your async functions - when you need to know all your parallel functions are complete (success or failure)
Takes a function to execute (executor) and a function to call on completion (onComplete).
executer is executed in the context of async-done, with all errors and results being settled.
onComplete will be called with a settled value.
Settled values have two properties, state and value.
state has two possible options 'error' and 'success'.
value will be the value passed to original callback.
MIT
