More changes for WinRT build

As mentioned in https://github.com/sharpdx/SharpDX/issues/334

Here are more DllImport Path changes, so both Windows 8.1 store and Windows Phone 8.1 pass the store certification. Each function tested in both application type.

Only last issue to have a common runtime is LoadPackagedLibrary which uses 2 different dlls in windows store an windows phone,so I'm not sure in which way to solve this one (additional build target  for one single string feels like a huge waste).