Rescale the support vectors in textmodel_svmlin() into probabilities for multiple classes

[stefan-mueller]

As discussed in #23, textmodel_svm() fails with a large number of training documents. However, it works when using the more powerful textmodel_svmlin() instead. Yet, currently, textmodel_svmlin() does not rescale the support vectors into probabilities for multiple classes. Adding this feature to textmodel_svmlin() would solve problems similar to #23.