Saving learner models in non-existing dir

The Learner.saveModel() method should probably check if the directory we are about to save in exists or not. Right now, it just throws an exception and dies, which can be annoying if you've just been training a big model. Similarly for the Learner.saveLexicon() method.

thanks for pointing it out, but I don't think I can do it in close future, if that's urgent ugh.

You can always reassign it :)