Update documentation to include instructions for running unit test cases.

[rakeshcusat]

As a contributor I don't know how to run test cases. So it is really hard to write new unit test cases and validate my changes. If someone provides the instructions then I am happy to update the documentation.

[rakeshcusat]

is the following command right to run the unit test?

python gcm/test.py

OR python -m unittest discover -s gcm -p test.py

The only problem with these commands is that I had to install the 'mock' manually before running the unit test.

I was thinking to do something similar to this

python setup.py nosetests -s