~ Initial SDL2 implementation

Limitation
- Dependent projects need to set NAZARA_PLATFORM_SDL2 if nazara has been build with SDL2 since OpenGL.hpp (and maybe some other headers) exposes platform details
- SDL2 window doesn't supports async window since the API isn't fitting for now
- Contexts parameters can't be changed until we close all the SDL windows (SDL limitation)

include/Nazara/Platform/WindowHandle.hpp

@@ -8,13 +8,17 @@
 #define NAZARA_WINDOWHANDLE_HPP
 
 #include <Nazara/Prerequisites.hpp>
-#if defined(NAZARA_PLATFORM_X11)
+#if defined(NAZARA_PLATFORM_SDL2)
+#elif defined(NAZARA_PLATFORM_X11)
 #include <xcb/xcb.h>
 #endif
 
 namespace Nz
 {
-	#if defined(NAZARA_PLATFORM_WINDOWS)
+	#if defined(NAZARA_PLATFORM_SDL2)
+	// Real type is SDL_Window
+	using WindowHandle = void*;
+	#elif defined(NAZARA_PLATFORM_WINDOWS)
 	// http://msdn.microsoft.com/en-us/library/aa383751(v=vs.85).aspx
 	using WindowHandle = void*;
 	#elif defined(NAZARA_PLATFORM_X11)