TST MiniBatchKMeans with many reassignments

Regression test for #3039.
DOC L1 distance works for sparse matrices

This was implemented in 3bc3d9f5ea773f6a2e9c207e9243102651785aac.
Updated examples to use pyplot for plotting instead of pylab

Some examples not updated because scripts throw error (probably not
because of the change)
Fixed pep8 violations. Some 'line too long' errors still remain.
Fixed some PEP8 violations even present in master branch, in examples/

This fixes some of the violations that were not caused by the change to pyplot
from pylab.
MAINT remove default values from private funcs in GMM

Removed default values for 'mean' and 'covars' arguments.
They did not have the excepted format causing the functions
to fail when called with default arguments.
ENH faster sampling in GMMs

A few optimizations for sampling with full covariance matrix:
1) use eigh instead of svd, which are equivalent (up to numerical precision),
2) use broadcasting instead of matrix multiplication for the product U s^(1/2),
3) simpler construction of the random vectors.
Merge branch 'pr/3120'

Fixes #3120, "Bad default arguments in gmm.py".
FIX numerical stability in GMM with eigh sampling
Merge pull request #3151 from rajatkhanduja/fixing_examples

Updated examples to use pyplot for plotting instead of pylab
adding conditional import to lfw.py for python3 support
Some more examples updated to use matplotlib.pyplot
Modified example 'plot_stock_market.py' to use matplotlib.pyplot

Modified despite #3153 because
- Error is not related to pylab or matplotlib.pyplot
- Removing the symbol 'DELL' from the example solves the problem
Merge pull request #3152 from jdowns/lfw-conditional-import

[MRG+1] adding conditional import to lfw.py for python3 support
Merge pull request #3154 from rajatkhanduja/fixing_examples

[MRG+1] Updated remaining examples to use pyplot for plotting instead of pylab
Extend utils.sparsefuncs: inplace scale and axis min/max