Fixed SelectKBest corner case: k=0

Without this fix k = 0 will have the same behavior as k = 'all' based on how
the array indexing was written.

I've included a test that demonstrates proper behaviour.
Merge pull request #3010 from griffinmyers/fix-select-k-best

Fixed SelectKBest corner case: k=0
FIX: make python setup.py clean also delete __pycache__ folders
FIX: remove casting warning under Python 3
FIX #3014: use a different folder for covtype data under Python 3
FIX: fix build under Python 2.6
BUG FeatureAgglomeration meaningless for no samples

The FeatureAgglomeration and WardAgglomeration estimators would run
when given a data matrix with no samples, but they would produce a
meaningless clustering.
FIX Makefile to use Perl again

Still fails on Mac. But at least the xargs is gone.
COSMIT: pep8, trailing spaces
Fix ValueError: Buffer dtype mismatch, expected 'INTP' but got 'long' on win-amd64
Merge pull request #3017 from cgohlke/patch-2

Fix ValueError: Buffer dtype mismatch, expected 'INTP' but got 'long' on win-amd64
FIX MSVC compile error C2036: 'void *' : unknown size

We were inadvertently using a GCC extension (pointer arithmetic on
void *). Fixes #2963, #3016.
TST: Fix ValueError: Buffer dtype mismatch, expected 'npy_intp' but got 'long' on win-amd64
TST don't run fast_dot tests on numpy>=1.7.2 + pep8
TST/COSMIT tests/comments for [sparse]center_data

Also removes sparse_std which was redundant
DOC: tfidf is actually tf*(idf+1) = tf + tf*idf
COSMIT: More readable drop for good test
FIX: checked in bad assertion in the last commit by mistake
Remove redundant yet unstable test_spectral_lobpcg_mode

The lobpcg mode is already tested in the test_spectral_clustering test.

LinAlgError was raised when executing test_spectral_lobpcg_mode on some
systems depending on the version of the scipy and LAPACK libraries.
TST: more robust test of dropping in Lars