Monitor file modification times

Background:

I have a process that is supposed to do some work continuously (EventMachine - read from MQ, do work, post result to MQ). For some reason this process is sometimes mysteriously failing. The memory usage or cpu usage seem normal .... but no real work is done. If I check the log file I see it is no longer growing indicating no work being done.

This pull requests adds the ability to monitor file modification times. So by monitoring the file modification time of the log file bluepill can determine the process is not really working anymore even when CPU and memory seem OK.