ensureAsync

[aearly]

Adds a new utility function for wrapping async functions. It ensures that the function always calls is callback on a later tick in the event loop. I'm also considering using this function internally to prevent stack overflows with synchronous iteration functions.

[coveralls]

Coverage increased (+0.25%) to 94.61% when pulling 7c7326b on ensure_async into f71193a on master.

[aearly]

I feel like there's a better name for this function, but going to use ensureAsync for now.