#29 Fix building and running under macOS

These two changes make ID2T build and run under macOS. The changes were also tested on Arch Linux to make sure nothing breaks. The change in build.sh was done by me, it introduces a new dependency (on GNU coreutils) which is easily installed via brew. The change in CMakeLists.txt was done by Roey. While the usual extension for dynamic libraries on macOS is .dylib, Python will only load .so libraries, so this workaround is necessary.