[MRG+1] Implementation of LinearSVR using liblinear code

Very initial implementation of LinearSVR using liblinear's code (which was always available, we just did not use it). Still lacking tests and documentation.
I know it would be inconsistent with ``LinearSVC`` but I feel this default is odd. I always set ``dual=False`` because if I had ``n_samples < n_features`` I'd use the non-linear method. But maybe I just have different use cases then you guys?