[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.
Loss can also be 'se'. You're missing a test for this case.

The mismatch between the loss names used by the binding and the python side makes the code hard to follow.

What I suggest:
- Accept `loss="hinge"` and `loss="squared_hinge"` in LinearSVC
- Accept `loss="epsilon_insensitive"` and `loss="squared_epsilon_insensitive"` in LinearSVR
- Replace 'ei' and 'se' by the above names throughout the code
- Use the above names rather than "l1" and "l2" in the docstrings (this is also more consistent with the SGD module)