Use StrongWolfeLineSearch implementation in LBFGS

Credits to Aaron Defazio: https://gist.github.com/anonymous/5439754

What's still needed is good default values for maxZoomIter and maxLineSearchIter.
I could also try to refactor StrongWolfeLineSearch to implement ApproximateLineSearch.
I think the Bracket case class in CubicLineSearch should be equivalent to the State case class in ApproximateLineSearch.