[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.
Rather not. This is a core part of the library and changing it is going to cause users' results to change. (Users like myself: I have code that depends on `LinearSVC` optimizing for the squared hinge loss.)