Python nose-ification and with coverage support

Hi,

This branch is interesting for 2 reasons:

(1) It tries to explain the code in lib/bb/COW.py and test it in lib/bb/tests/test_cow.py

(2) Uses nose and coverage modules to be make it easier to write new unit test cases for bitbake and possibility increase the quality and stability of the tool

Code has been tested with a real image build.