add PyYAML requirement

[mboersma]

Fixes this error when installing from source as suggested in the README:

$ python kubefuse/kubefuse.py ~/k8s
Traceback (most recent call last):
  File "kubefuse/kubefuse.py", line 10, in <module>
    from client import KubernetesClient
  File "/Users/matt/Projects/kubefuse/kubefuse/client.py", line 2, in <module>
    import yaml
ImportError: No module named yaml

After adding PyYAML, I'm able to run kubefuse fine in a virtualenv with python2.7.

Refs #6.

[bspaans]

Awesome, thanks! :+1: