Add dependecy checking into async.auto

[benzen]

I almost exclusively use async.auto, so maybe this apply to other I can't tell.

It append frequently that i make a async.auto block, refactor it, that thing break because of a savage renaming.

It would be nice if async call final block with error if the dependcy graph is broken. Or throw an exception if there is no final function

async.auto
  "a": (cb, results)-> cb null, "a"
  "c": ["b", (cb,results)-> cb null, "c"]
  (err, results) -> 
    if err then console.log err

This would produce a message like this for exemple:

Depency is unsatisfied "c" expect "b" but no declaration where made for "b"

[aearly]

#663 seems to fix this.

[benzen]

Yop great