Windows Phone 8 function to rate an App. Add this near the bottom of your MainPage class.

rateApp.cs

	/// <summary>Called by Lua when it is requesting a login popup to be shown.</summary>
	/// <param name="sender">The CoronaRuntimeEnvironment that dispatched this event.</param>
	/// <param name="e">Provides the Lua vent table's fields/properties.</param>
	/// <returns>Returns a boxed object to Lua.</returns>
	private CoronaLabs.Corona.WinRT.ICoronaBoxedData rateApp(
		CoronaLabs.Corona.WinRT.CoronaRuntimeEnvironment sender,
		CoronaLabs.Corona.WinRT.CoronaLuaEventArgs e)
	{
		MarketplaceReviewTask marketplaceReviewTask = new MarketplaceReviewTask();
		marketplaceReviewTask.Show();
		// This returns nil to Lua.
		return null;
	}