Fix citation in TfidfTransformer

Corrected names order for tf-idf citation and corrected pages numbers.
See: http://nlp.stanford.edu/IR-book/pdf/irbookprint.pdf
Merge pull request #2877 from pmandera/tfidf-citation

Fix citation in TfidfTransformer
ENH: small optimization of the classification criterion
ENH: small optimization to the regression criterion
ENH speed up RBM training with scipy.special.expit

SciPy 0.10 already has an implementation of the logistic function called
scipy.special.expit.

Using this makes RBM training 12% faster, as measured by observing the time
per iteration as reported by the plot_rbm_logistic_classification.py example,
disregarding the first iteration as an outlier (it's consistently faster than
the rest, not sure why). The speedup is in the expit function, not the
inplace operations; those are there to make benchmarking easier, but they
certainly won't hurt.
ENH re-instate extmath.logistic_sigmoid

For backward compat only, so immediately deprecated.
FIX DictVectorizer handling of empty inputs

Fixes #2883.
TST older nosetests compat in DictVectorizer test
putting cluster  on new line to solve sphinx documentation formatting issue
Merge pull request #2882 from larsmans/expit

Speed up RBM training with scipy.special.expit
Merge pull request #2884 from dsullivan7/clusterdoc

Sphinx formatting issue for Coclustering and Biclustering
[TYPO] s/migh/might/
Update supervised.py
Update supervised.py
Merge pull request #2895 from kaushik94/master

 supervised.py with updated documentation
Update plot_forest_importances.py

import statement has moved to the top of the file
Merge pull request #2897 from ugurthemaster/patch-1

Update plot_forest_importances.py