Add .DS_Store to gitignore

Added .DS_Store filename to gitignore so that editing on Mac (and
browsing files using Finder) doesn't add annoying and useless files.
CMake adjusted for OS X and homebrew

Fix for issue #4.

hint for fix:
http://stackoverflow.com/questions/9898643/cmake-include-directories-ord
er-after-before
Revert "CMake adjusted for OS X and homebrew"

This reverts commit 0f2fd713dee437a1d944f2df0329dc00e9016023.