ENH: add PolynomialFeatures preprocessor
address PR2585 comments
import pylab -> import matplotlib.pyplot
TST: fix PolynomialFeatures test error
TST: fix doctests & pep8
DOC: adjust notes in PolynomialFeatures
reorder polynomial features
fix ordering & docs of PolynomialFeatures
TST: remove stray print statement
DOC: typo in kernel formulas
Merge pull request #2608 from jakevdp/master

DOC: typo in kernel formulas
Merge pull request #2585 from jakevdp/polynomial

[MRG] ENH: add PolynomialFeatures preprocessor