Add nibabel dependency to setup.py

Needed to change the way __version__ works because dependencies are checked in nilearn/__init__.py
so that setup.py could not be executed when nibabel was not installed
Remove nibabel section from the install instructions

now that nibabel is an explicit dependency in setup.py
Use exec instead of deprecated im.load_source
Merge pull request #310 from lesteve/add-nibabel-dependency-to-setup-py

Add nibabel dependency to setup.py