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.
Any chance you can add docstrings? This is [based off sphinx-y formatting](https://pythonhosted.org/an_example_pypi_project/sphinx.html#function-definitions)

(I'm only ~35% sure about this, please correct me where I'm off :)

``` python
"""Set the CPU Affinity for the current process.

:param cpu_affinity_csv: Which CPUs to pin this process to
:type cpu_affinity_csv: string with comma separated values
"""
```