CB-8750 [wp8]: Allow resolution-specific splashscreen images

There are two aspect ratios for Windows Phone devices: 15:9 and 16:9. At
the moment, though, there is only one splash screen by default for WP8
Cordova apps. When stretched to fit all possible resolutions, this can
sometimes look bad.

WP supports resolution-specific splashscreens, so this commit adds the
same support to the splashscreen plugin. It uses the same base name for
the splashscreen, but adds modifiers such as ".screen-<res>" based on
the device. If a screen-specific image is found, it is used. If not, we
fall back to the single default image.

Fixes https://issues.apache.org/jira/browse/CB-8750

github close #41
github close #42

plugin.xml

@@ -58,7 +58,7 @@
 
         <source-file src="src/android/SplashScreen.java" target-dir="src/org/apache/cordova/splashscreen" />
     </platform>
-    
+
     <!-- ubuntu -->
     <platform name="ubuntu">
         <header-file src="src/ubuntu/splashscreen.h" />
@@ -98,6 +98,7 @@
         </config-file>
 
         <source-file src="src/wp/SplashScreen.cs" />
+        <source-file src="src/wp/ResolutionHelper.cs" />
 
     </platform>
 
@@ -111,6 +112,7 @@
         </config-file>
 
         <source-file src="src/wp/SplashScreen.cs" />
+        <source-file src="src/wp/ResolutionHelper.cs" />
 
     </platform>