Preliminary support for probability estimate in SVM.

Added method prob_predict() that does probability estimates. It's only
tested for SVC (classification), so it's untested for SVR/OneClass.

To be able to do probability estimates, you have to specify parameter
probability=True (now on by default). Turning off this parameter might
speed up calculations, althougth I did not notice any significant
difference.

git-svn-id: https://scikit-learn.svn.sourceforge.net/svnroot/scikit-learn/trunk@658 22fbfee3-77ab-4535-9bad-27d1bd3bc7d8