Use liblinear in SVM module when kernel is linear.

In the future, a switch library='auto' should be implemented in case
one wants to use libsvm also for linear kernels.

BTW, this breaks (cosmetically) some examples and tests.

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

git-svn-id: https://scikit-learn.svn.sourceforge.net/svnroot/scikit-learn/trunk@662 22fbfee3-77ab-4535-9bad-27d1bd3bc7d8