added Process.get_exitcode method

Hi Daniel,

I've added the get_exitcode method mainly because I needed a way to tell if my process started with Process.create is still running or not. The usual way of calling Process.wait or Process.waitpid fails as soon as one connectes stdin, stdout or stderr with pipes or files. It seems that wait doesn't recognize the process as a child process (ECHILD). Not sure how they try to figure this out internally...

Martin