Copy assets to build output path to fix issue #26

Fix to the CMake script to actually copy the "data" directory
to the build output directory where the eden executable will
be placed. This change has been tested with Xcode and Unix
Makefile generation and builds/executes well using both.

Also removed the extra parameters from the ELSE/ENDIF calls
in the script, as they haven't been needed for several versions
of CMake now.