ENH semisupervised learning in Naive Bayes (EM algorithm)
ENH semisupervised text classification demo

Based heavily on ordinary text classification example
ENH only re-estimate unlabeled samples in EMNB

As per Nigam et al. 2000.
ENH start EMNB from supervised model instead of zeros
ENH optimize EMNB: cache X and Y slicing (five-fold speed increase)
COSMIT rename EMNB to SemisupervisedNB

Less cryptic than a four-letter acronym
ENH relabel option to SemisupervisedNB
BUG fix Naive Bayes fit with non-empirical prior
BUG swap E and M steps in SemisupervisedNB
ENH speed up EM for NB 10x by using linalg.norm
DOC add SemisupervisedNB to NB narrative docs
give up on EM for Gaussian NB for now
ENH stratified label deletion in semisup example + stricter tol in EM NB
TST EM for NB + add check for BaseDiscreteNB
DOC point out similarity with self-training at @amueller's request
DOC SemisupervisedNB in classes.rst
Merge branch 'master' into emnb

Conflicts:
	sklearn/preprocessing/__init__.py
COSMIT naive_bayes pyflakes-clean
DOC link SemisupervisedNB
COSMIT pep8