Datasets refactoring.

Do not return dictionary on the datasets. We still have to define
a common API for all datasets, but in the meantime this will allow
us to have some clean examples that will hopefully lead to a cleaner
dataset implementation.

From: Fabian Pedregosa <fabian.pedregosa@inria.fr>

git-svn-id: https://scikit-learn.svn.sourceforge.net/svnroot/scikit-learn/trunk@412 22fbfee3-77ab-4535-9bad-27d1bd3bc7d8