updates while reductionizing matrix factorization

mf.cc is the reduction-friendly matrix factorization code, which is currently in development. it's included here but not actively called by vw.

gd_mf.cc is the old matrix factorization code, currently activate.

mf.cc is currently about 3-4 times slower than gd_mf.cc. in debugging this, john and i templatized several functions in gd.cc for performance gains and cleaned up some code.

a predict function has been added to the reductions interface as well. default behavior at the moment is simply to call the learn function.

some stderr test results have changed slightly, as zero weight training examples are now considered for feature re-scaling.