* adds shadow opacity, shadow color, shadow offset, shadow radius to navbar for ios

* mentions ios shadow in styling-the-navigator docs

* Add paddingTop to android ToolBar.

* Update documentation

* add support to tell screen to place its self underneath the statusbar for android.

* added property to documentation.

This commit saves navBar component props in propsRegistry and restores the saved props
when the component is mounted, allowing us to pass unserializable props to custom navBar component.

Until now, buttons would use the same font family used by the title,
this commit introduces navBarButtonFontFamily property which allows to set
a different font family for title and buttons.

* Support subtitle FontFamily on size on Android

Font family can be either a .ttf/.otf asset or one of the default font families:

* sans-serif (regular)
* sans-serif-light
* sans-serif-condensed
* sans-serif-thin
* sans-serif-medium

* update docs

* add disabledButtonColor in order to set navigation button disabled color

* add disabledButtonColor to the docs

Ability to set a flat border under the TopBar. `topBarElevationShadowEnabled` should be set to true if using this feature.

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

added a missing comma in the 'style object format' so to help users who copy/paste this code block

Some navBar button properties were marked as common by mistake.

Separated iOS and Android only properties.

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

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

* Add documentation.

Reading the docs I couldn't see a way to simply change the color of the status bar on Android. Only via https://github.com/wix/react-native-navigation/issues/571#issuecomment-267255350 I saw there was a prop to do this.

(Assuming this is Android only since iOS only allows for light or dark).

* Initial commit

* Initial commit

* Clean-up title