Turn off probability estimates by default.

This was done for performance reasons. Also add documentation on this
parameter in the docstring.

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

git-svn-id: https://scikit-learn.svn.sourceforge.net/svnroot/scikit-learn/trunk@659 22fbfee3-77ab-4535-9bad-27d1bd3bc7d8