[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.
We could easily add an `"auto"` setting that checks `n_samples` and `n_features`.

If both `n_samples` and `n_features` are large, you still want linear models because they're fast and harder to overfit.