Implement basis for osx version using psutil library

This is the start for implementing the OSX agent/client. It turns out that fetching cpu time and memory usage for a process through python on OSX is not trivial. One option would be to parse **ps** return. But this is slow. Instead I found a portable python package that does the job for us. It is called [psutil](http://pythonhosted.org/psutil/). The cool part of using this library is that we can delegate platform support management. This would simplify our codebase. I propose we use psutil on all platform.

Another point to note, on OSX it is not possible (as far as I know) to get the exit code for a process unless you catch it while terminating. So I did not implement ProcessMonitor.exit_code() method you have on Windows. I did not understand its purpose too :(. Was it only for testing purpose?

**Bonus**: Integrated _pylintrc_ configuration file so we can push default parameters locally and on Travis
Indeed.  If my understanding of POSIX is correct, the proper flow would include waiting for a signal from the process, then invoking a blocking call to `waitpid()`.  However, for this to work correctly, the monitored process needs to be a child of the monitoring process...  We could always add a `ptrace()` option sometime to take control of the process, but that doesn't seem like a very nice thing to do.