add eval

[fibo]

use eval as a last fallback for not resolved tasks eval(eval('function (){return' + 'task not resolved' + '}'))

[fibo]

eval is evil :)

If a task is not resolved, dflow engine will throw, that is the right behaviour.

However, I implemented arrow functions!!!!! https://github.com/fibo/dflow/blob/master/src/engine/inject/arrowFunctions.js