add a "safe" shebang to pyside_postinstall.py

This shebang works, even with DOS line endings on OSX.
Without this added shebang, the niceness of having this script in the bin dir
becomes really annoying under Mavericks, because it then becomes pretty hard
to run the post install script explicitly.
Merge pull request #24 from ctismer/master

add a "safe" shebang to pyside_postinstall.py