Use preferred callback refs

[gojohnnygo]

This PR implements preferred callback refs (vs string refs) per React Refs documentation.

[coveralls]

Coverage decreased (-1.8%) to 89.871% when pulling e8bcab8 on gojohnnygo:RefCallback into e19bce3 on JedWatson:master.

[coveralls]

Coverage decreased (-1.8%) to 89.871% when pulling 38ca948 on gojohnnygo:RefCallback into e19bce3 on JedWatson:master.

[coveralls]

Coverage decreased (-1.1%) to 90.608% when pulling 15ba7d2 on gojohnnygo:RefCallback into e19bce3 on JedWatson:master.

[JedWatson]

Thanks @gojohnnygo, this looks good!