Visual Studio 2013 targets Windows 8.1. You need to use Visual Studio 2012 to target Windows 8.
But you can also have a look to this post : Is it possible to create a Windows 8 Store App from Visual Studio 2013?
By manually modifying the .csproj file it seems possible to use 2013 to target 8.0. Remove the lines :
<TargetPlatformVersion>8.1</TargetPlatformVersion>
<MinimumVisualStudioVersion>12</MinimumVisualStudioVersion>