Splash screen support via config.xml for WP8 and Windows8

https://issues.apache.org/jira/browse/CB-6308
https://issues.apache.org/jira/browse/CB-6309
Adds support of 'SplashScreen' preference for WP8 and Windows8.

    <preference name="SplashScreen" value="SpashScreenImage-Default.jpg"/>
    <platform name="wp8">
        <preference name="SplashScreen" value="assets\SplashScreenImage-WP8.jpg"/>
    </platform>
    <platform name="windows8">
        <preference name="SplashScreen" value="assets\SplashScreenImage-Windows8.png"/>
    </platform>