add subtitle support to navigation bar

This is needed since some of the data we need to send in the events
can not be serialised and added to a Bundle.

When Snackbar was displayed while another Snackbar was displayed,
the last ref to the current Snackbar was removed and it didn't get removed
when onScreenChange was called.

* Animate actions
* Animate fab and actions with Snackbar

Update react-native to 0.31.0

fixes #274

* Created Snakbar class to encapsulate Snackbar logic.
* Clicking the action now emits event to current screen
* Action color works