expose children in AsyncCreatable.js

[gwyneplaine]

This PR ports over the fantastic work that @DanielHeath has done to refactor AsyncCreatable to expose the wrapped Select component as a child (as we do in Async and Creatable respectively).

[coveralls]

Coverage decreased (-0.6%) to 92.248% when pulling b7e876d on fix/expose-select-to-async-creatable into a1e1069 on master.

[coveralls]

Coverage decreased (-0.01%) to 92.824% when pulling e5b1c66 on fix/expose-select-to-async-creatable into a1e1069 on master.

[JedWatson]

re: coverage, this change removes unnecessary code that was previously covered, so the change is due to having less code overall. feel free to ignore.