* Due to race condition when showing modal, buttons were added to
previous Toolbar.

* Minor refactor, moved `updateStyles` from BaseReactActivity to
StyleHelper class since it's now also used by RnnModal

Minor refactor after merging #52.

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

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

Navigation views such as ToolBar, StatusBar, NavigationBar can now be
customized through a static JSON defined in each screen component.

Styling options introduced in this commit:

 - ToolBar color
 - StatusBar color
 - NavigationBar color
 - Buttons tint color
 - Title color
 - Tab normal text color
 - Tab Selected text color
 - Selected tab inidicator color

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.