Signal queueing mechanism

[sclasen]

provide a managed data structure accessible from the fsm context that allows you to defer handling a signal until you are in a latter state (or later portion of a composed decider)

/cc @fabiokung @dgouldin

//enqueue a signal you want to handle later
ctx.Enqeue("SignalName", input)

var signals []interface{} //provide typed mechanism
signals = ctx.Queue("SignalName")

//composed decider variant
fsm.QueueSignal("SomeSignalName")