Requirement Cleanups

All of Nikolas basic requirements can be found in setup.py. So ``mock`` is removed from there because it is only needed for testing.

In addition the requirements files have been cleaned from the packages already present in setup.py to have less duplication. This should make it easier for someone wanting to contribute to Nikola to find the right place for his dependencies ;)
And there is a switch made from fluggos Pillow repository to the official one as this one now has fluggos changes merged (and therefore Py3k-support \o/). Hopefully we will see a new Pillow release soon because this would give us the ability to only have one requirements-file for both Python versions and making the ``install_requirements.py`` file obsolete.

Installs with ``python setup.py install` or ``pip install .`` still work the same.