ENH refactor squared-norms computation to extmath

k-means can now use the memory-efficient dense implementation from
metrics.pairwise, while pairwise can use the fast sparse implementation
from k-means.
ENH use row_norms in KDE code

There may be other places where it is useful.
FIX fit followed by partial_fit in multiclass SGD

Fixes #2496 and includes a non-regression test. Since the introduction
of fast_dot (and NumPy 1.6), dot products between arrays of various
memory layouts have become fast enough to no longer require storing coef_
in Fortran order. The only noticeably slower operation is multiplying a
CSR matrix with a Fortran-ordered array, but then building the CSR matrix
in the first place is more likely to be the bottleneck.
FIX: clean-build target does no longer exist
MAINT use subprocess.call, not os.system

The former is safer and faster.
COSMIT remove dead code in k-means

Private functions _k_init and _kmeans_single were doing too much work.

Should improve the relative coverage.
Merge branch 'refactor-squared-norms'