Fix build of SDL2 sample on OSX

This corrects the build errors I receive when trying to build the SDL2
sample on OS X. This occurs due to the wrong find_package helper
scripts being used for SDL2 & friends -- the CMake-distributed
FindSDL.cmake && FindSDL_image.cmake is intended for use only with
SDL version 1.

There exists no "official" CMake find helper script at the time of
this writing (CMake v3.0.2) for either SDL2 or SDL2_image, so custom
helper scripts, FindSDL2.cmake and FindSDL2_image.cmake has
been added to the project's local cmake modules directory.