TopTabParams are a subset of ScreenParams, moved the properties that
are shared by both classes to BaseScreenParams.
This change was needed to report screen changes properly in ViewPagerScreen

Remove react hacks

This commit removes necessary hacks which were introduced due to
limitations of previous React Native apis.

Mount/unmount logic isn't coupled to view attached/detached
to window callbacks. Instead, `unmountReactApplication` was added.
We can safely call it when a screen is popped and when application is
reloaded in debug.

* Don't show fab of a screen that's pushed to a none active tab
* Change fab when navigating back in the stack with back button
* Add screens before SnackbarAndFabContainer

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