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
MAINT: Fix missed usage of deprecated tools.rank
DOC: Add links for build machines. Closes #1153 [skip ci]
Merge pull request #1154 from jseabold/testing-machine-links

DOC: Add links for build machines.
Merge pull request #1196 from kain88-de/fast_fft_acf

ENH: ensure O(N log N) when using fft for acf
COMPAT: Correct bit_length for 2.6
DOC: Update comment to reflect code. [skip ci]
Merge pull request #1545 from jseabold/rank-cleanup

MAINT: Fix missed usage of deprecated tools.rank
Merge pull request #1547 from jseabold/fix-bit-length-2.6

COMPAT: Correct bit_length for 2.6
ENH: Medcouple for robust skewness estimation
ENH: Function allowing other functions to be applied using only 1d
    at a time
ENH: Robust Skewness from Kim and White
ENH: Robust Kurtosis from Kim and White
ENH: Helper functions for quantile.  Different from numpy.
ENH: Tests for medcouple
BUG: Made use of np.apply_along_axis and replaced custom version
General improvements to robust skewness and kurtosis
ENH: Generalized other texts to work along axis for standards
Simplified code to remove new weighted quantile function, instead
    use np.percentile
Removed incorrect test file in tools/tests/test_statstools.py
Added tests for multi-axis functions in stattools
Clean-up and documentation for robust skewness and kurtosis measures
Cleaned-up stattools
Added docstring for those missing
Added test for robust kurtosis
Fixed defect where ties were not handled correctly by medcouple
Reverted space additions around ** operator
Removed unnecessary float powers
100% test coverage

Conflicts:
	statsmodels/stats/tests/test_statstools.py
Revert auto-spacing
ENH: Added flag to select excess or not
Allows raw values to be returned when excess=False

Conflicts:
	statsmodels/stats/stattools.py