PidExecutor - an executor that waits for the specified file to exist

This is an executor that starts a process, then waits for a specified file to come into existance. Usefull for integration tests, etc.
It depends on what way you look at it. This way it focuses more on the original process. I also wanted to keep it in line with the existing documentation. But I suppose it would be best to have both of those views reflected in the comment.