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.