REF: ensure O(N log N) when using fft for acf

The numpy implementation will fall back to a O(N**2) version of the fft
in cases where it cannot factorize the array length, which can be a
increase in runtime of several order of magnitude. This uses the
_next_regular function from scipy to ensure that we always use array
lengths for which fftpack is optimized.
TST: add test for _next_regular

also copy the test for _next_regular from scipy
ENH: accept non-int definitions of cluster groups
ENH: Option to avoid "center" environment.
ENH: use np.unique for indexing clusters
STY: import pandas as pd
BLD: exclude sandbox in relative path, not absolute
Merge pull request #1458 from TomAugspurger/sandbox-build

BUG/BLD: exclude sandbox in relative path, not absolute
DOC: Remove defunct argument. Change default kw. Closes #1462.
Merge pull request #1452 from jseabold/pandas-imports-examples

STY: import pandas as pd
Merge pull request #1463 from jseabold/fix-1462

DOC: Remove defunct argument. Change default kw. Closes #1462.
MAINT: Fix overflow field-name cells in docs.
MAINT: Be more judicious about whitespace in parameters, etc.
MAINT: Update notebooks CSS
Merge pull request #1468 from jseabold/css-fixes

MAINT: CSS fixes
DOC: Fix bad links.
Merge pull request #1469 from jseabold/fix-json-links

DOC: Fix bad links
DOC/BLD: Log errors in executed cells.
DOC/BLD: Add make notebook
Merge pull request #1470 from jseabold/nbgenerate-exceptions

DOC/BLD: Log code exceptions from nbgenerate