BUG don't densify sparse matrix in BernoulliRBM.score_samples

Also documented the non-determinism of this method.
ENH speed up progress reporting in RBM

Previously took up to 45% (!) of the time in training, not the 10% promised
in the docstring, for the digits example. Now down to at most 3%.

Pseudo-likelihood is now reported for the entire X, rather than batch-by-batch,
which will take more memory.
DOC fix BernoulliRBM._fit docstring

Return value is gone now. Thanks @jnothman.
Merge pull request #2642 from larsmans/rbm-speedup

[MRG] faster convergence reporting in RBMs