Updated RecastDemo to SDL2

* Renamed a bunch of variables in main.cpp to be more descriptive.
* Removed unnecessary SDLMain.h and SDLMain.m OSX objective-c class as well as the plist, strings, xib and icns files, which are not needed.
* included cstdio in imguiRenderGL since SDL2 doesn't do it for us.
* Updated premake5 script to support SDL2 and to set the debug directory, as well as copy the SDL2.dll to the target directory on Windows.
* Updated readme with more descriptive, platform-specific demo project setup instructions
* Updated appveyor build script to build vs2015 as well.
* Updated Travis build script to build SDL2 from source, because they use Ubuntu 12.04 which doesn't have the libsdl2-dev package in its repositories.

RecastDemo/Source/Sample_TileMesh.cpp

@@ -22,6 +22,11 @@
 #include <string.h>
 #include "SDL.h"
 #include "SDL_opengl.h"
+#ifdef __APPLE__
+#	include <OpenGL/glu.h>
+#else
+#	include <GL/glu.h>
+#endif
 #include "imgui.h"
 #include "InputGeom.h"
 #include "Sample.h"