Mutation observers

[calvinmetcalf]

add mutation observers as an option

[calvinmetcalf]

while we were at it disabled the fallback to the real set immediate as it would seem that this is broken in ie, see cujojs/when#197

[calvinmetcalf]

The generational event queue i what I was calling in my head the modified queue from 73bc498 basically we assure all things we set immediately are never executed in the same event loop. I called it generational because all the events set in one event loop will be executed in the next event loop. I.E. things created in loop 1 are executed in 2, and anything in 2 is in 3 etc.