Merge in QP-ADMM for W-step

Added qp_admm.m and proj_simplex.m under util
Using threshFull_mixed (the closed form proximal operator)
Initialized RHO to 0.1 for each call to mlr/rmlr ADMM (used to be static)
Increased default ADMM_STEPS to 20 (to compensate for static RHO)