Added psutil to check if Steam is running when starting Ice.

A very basic and quick check to see if Steam is running when starting Ice.
It uses psutil to loop through all currently running processes and searches for "steam.exe" under Windows and "steam" under GNU/Linux. Still need to find out what the process name under Mac OSX is.

The implementation uses try, except to import psutil, so if it isn't installed it will still work, but it just won't be able to check if Steam is running.