ENH 10x speedup in dump_svmlight_format

As measured by dumping

X = rand(10000, 4000, density=.0125)
y = np.random.randint(0, 5, X.shape[0])

to /dev/null.
DOC: NMF narrative: describe optimization problem
Merge remote-tracking branch 'upstream/master' into sparse_labelbinarizer
FIX _fit_binary