Convert Windows Phone 7 projects to Windows Phone 7.1 format and fix Windows Phone 8 build

The projects for the WinPhone7 Client and AdRotator.Examples.WinPhone7 were both set up as Windows Phone 8 apps, these commits change the format of the project to target Silverlight on WP7 and update the NuGet references to point to their WP7 counterparts.

Furthermore the Windows Phone 8 project referenced a missing GoogleAds.dll this has been corrected to reference Google.AdMob.Ads.WindowsPhone7.dll which is present in the WindowsPhone8 libraries folder.

The set of Capabilities defined in the Windows Phone 8 WMAppManifest had selected all capabilities which caused the app to quit at the splash screen, this pull request also sets the capabilities to the set that is required to run the application.