Use git tag version for engine's version string

This changes the default engine version string from "custom" to a
much more useful string for us to use. This effects the resulting
output from Rocket::Core::GetVersion() and the version displayed in
the visual debugging tool.
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 incorrect find package helper
script being used for SDL2 -- the CMake-distributed FindSDL.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), so a custom helper script,
FindSDL2.cmake [1], has been added to the project.

1. Source: https://code.google.com/p/freerct/source/browse/trunk/CMake/FindSDL2.cmake
SDL2 sample: Add key binding for toggling visual debugger