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

Slightly more backwards compatibility in android leftButtons

When more then one modal was displayed, and dismissAllModals
was called, the top most modal was dismissed until no modals
were visible. This resulted in quirky animation since user saw
all modals getting dismissed.
This commit simply changes to order in which modals are dismissed to
avoid this bug.

This commit closes #224 and fixes #252. Calling `setButtons` with
`leftButton` object still works for now but is deprecated and will be
removed in future version.