Added execute bit for cmake_clean.sh.
Updated documentation on build option being cached.
Corrected usage of message() command for showing CMake status.
Generated sources are defined before normal sources.

To be consistent with the rest of the CMakeLists.txt.
Support more source file extensions.

Added support for: .mm (Objective-C++ source code) and .s (preprocessed assembly source code).
Renamed IOS and Mac OS X framework macros.

These macros may be used later not just to setup frameworks but also
other linker flags.

Renamed setup_ios_frameworks    -> setup_ios_linker_flags.
Renamed setup_macosx_frameworks -> setup_macosx_linker_flags.
Recreate "assets" directory back in Git repository.

Git does not handle empty directory like SVN. A directory in Git must at least contain a file to be tracked.
Added .gitignore to do the work and at the same time tell Git to ignore all the symlinks/files in this directory.
Merge local branch.