Toolbar buttons not updating
Pushes into BottomTabActivity not always working
Modal not closing properly when back pressed

Removed propTypes from example-redux screens

Adds BottomTabActivity which replaces TabActivity
Adds a dependency on `com.aurelhubert:ahbottomnavigation:1.2.3`
Uses iOS tabsStyle, with additional parameter that allows toggling of whether to show the titles of inactive tabs

Also changed the structure used to store Modals
from a Map based on navigatorId to a stack.
Reason for the change is that screens can be
pushed only to the topmost Modal.

This commit also fixes issue where style was not cleared when changing
tabs, and when pushing or popping screens.

From now on, calling start*Application will clear the activity stack and the new activity as
the root of an empty stack. This means we don't support "Modal" or opening screens in new activity.

Enabled resources (Toolbar icons) in release mode.

Navigator.push allows us to open a new screen. Currently it starts a new Activity with the screen
as it’s root view, but this is only a temporary behaviour. The intended behaviour is similar to
View Controllers on iOS which allows you to manage view hierarchies or a stack of fragments
managed by FragmentManager on Android.

The approach we took is fairly simple. Navigation is created natively in corresponding activity,
currently only TabActivity is supported, and we pass the styling parameters and screenId to it.
Each screen is rendered in its own ReactRootView and is wired to the same ReactInstanceManager,
so all screens share the same java script context.