Allow setting nice level and proc affinity w/psutil

@Yasumoto @rgs1 

Adding psutil as a requirement, and using it to set niceness level and CPU affinity. CPU affinity will allow us to pin the process to a specific core as scapy can be CPU intensive at high packets per second load. In production, this will let us sandbox the monitoring code away from the Zookeeper daemon itself.
Not that you should fix this in this round, but I think we [should use mock](http://mock.readthedocs.org/en/latest/) to enable tests on travis. We (the zktraffic-community) can also have a test suite that can be run when not on travis as well, but let's get this in and pick that up later.