fixes to new_mf

fixes for new_mf, including new interface.

rank is now specified with ``--new_mf K`` instead of ``--rank K --new_mf``. ``--rank`` is reserved for gd_mf.

reasonable performance on movielens 100k with these options:

    vw -b 22 -q ui --new_mf 10 --passes 50 -k --cache_file movielens.cache --l2 1e-6 --learning_rate 0.5 -f movielens.reg --sgd

adaptive, normalized, and invariant seem to have inferior performance at the moment.