[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.
BTW I just realized that by default LinearSVC uses the squared hinge (loss='l2') instead of the hinge loss (loss='l1') which is extremely confusing IMHO. It  was already discussed here (http://sourceforge.net/p/scikit-learn/mailman/scikit-learn-general/thread/CAD-7WjrjJEPx4%2BMSBacYLuBDo_7yK5VMBsHdWdV-VqguYK5Z%2BQ@mail.gmail.com/) but maybe this is the occasion to change this default behaviour ? ping @larsmans @GaelVaroquaux @agramfort