[iOS] Added ability to set static(root) background image

added 03d380f8a9 back and also applied this fix https://github.com/wix/react-native-navigation/issues/428#issuecomment-278268928 on top of it

* Fix disabledBackGesture.

This `return self._disableBackGesture ? self._disableBackGesture : YES;` check is incorrect, it's the same as `true ? true : true;`.
Also `_disableBackGesture` was never set. We should just `return disabledBackGestureBool` (bool value of the prop).

* [iOS] Enable back swipe gesture when a navBar is hidden. Resolves this issue: https://github.com/wix/react-native-navigation/issues/930

added 03d380f8a9 back and also applied this fix https://github.com/wix/react-native-navigation/issues/428#issuecomment-278268928 on top of it

* Revert accidental commit: 3cdb1b37f6

Add setSubTitle implementation for iOS + docs

Custom title view in navigation bar support in `navBarComponentAlignment:'fill' | 'center`

* Add ability to set attributes of navigation bar buttons separately.

* Added ability to set button style when calling `setButtons` method.

* Add documentation.

* Adds iOS callback for viewWillAppear/didAppear e.t.c.

* Adds in screen events on Android

* Fixed issue with rightButtons getting ignored in preference for empty Screen buttons

* Android screen lifecycle

This commit is contains a few changes and enhancements to the PR made by simon:

* Use getEventEmitter instead of sendNavigatorEvent
* Dispatch willDisappear and didDisappear events when pushing screens
* Dispatch willAppear and didAppear events when popping screen

fixes #800 #797

* Fixed issue with rightButtons getting ignored in preference for empty Screen buttons

* Fixes setting transparency of navigation bar using setStyle or when no transitionCoordinator

* Set to use old bridge for compatibility with 0.30+

* Fixed crash occurring when tabBarBackgroundColor or tabBarButtonColor is not supplied

* Adds setStyle property to index.js

* Adds native code for setting style on RCCViewController

* Fixes setStyle function

* Fixes setting style on navigator

* Adds some styling to tab bar controller and fixes some bugs

* Adds ability to set modal presentation style

* Makes sure autoAdjustScrollViewInsets isn't propagated through when push another view controller

* Changes import to reference controllers rather than react-native-controllers and fixes setStyle mutating colours

* Fixed to work with RN 33

* Fixes updating title font styling with new RCCTitleViewHelper logic

* Fixes subtitle being shown

* Adds extendedLayoutIncludesOpaqueBars to navigation styling

* Adds ability to add shadows to text in navigation bar / tab bar

* Fixed merged ReactGateway.java file

* Fixed issue with rightButtons getting ignored in preference for empty Screen buttons

* Fixes compiler error

* added status bar color scheme for single screen which isn't pass on push. Set "statusBarTextColorSchemeSingleScreen" on the navigatorStyle

* make statusBarTextColorSchemeSingleScreen preferred option when both statusBarTextColorScheme and statusBarTextColorSchemeSingleScreen are exists

* To avoid white flash while component loads