CB-6308 WP8. Add Splash Screen img support via config.xml preference

https://issues.apache.org/jira/browse/CB-6308
Adds support of 'SplashScreen' preference for WP8.

    <preference name="SplashScreen" value="assets\SplashScreenImage-WP8.jpg"/>

or 

    <preference name="SplashScreen" value="SpashScreenImage-Default.jpg"/>
    <platform name="wp8">
        <preference name="SplashScreen" value="assets\SplashScreenImage-WP8.jpg"/>
    </platform>