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
I thought the purpose of the `get_memory_usage()` module-level function was to facilitate mocking, but it doesn't seem like it's mocked in the tests.  Maybe you can simply move its implementation here?