added the btNNCGConstraintSolver, based on the paper "Nonsmooth Nonlinear Conjugate Gradient Method for interactive
contact force problems". The solver needs a lot of iterations, before the quality goes up (~ 1000)
Thanks to Gabor PUHR for the contribution!
Improved the btLemkeSolver.
Remove the sparse optimizations from the btMatrixX.h, replace it with explicit call to rowComputeNonZeroElements (only used in the btSolveProjectedGaussSeidel), it was likely slowing things down, without being useful.
Re-enable SIMD in the solver (was accidently disabled in Bullet 2.82 release)

git-svn-id: http://bullet.googlecode.com/svn/trunk@2709 08e121b0-ae19-0410-a57b-3be3395fd4fd