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
ENH: using human-readable group names instead of integer ids in MultiComparison
added file that got left behind
Put back an import that my IDE incorrectly flagged as unused
Added unit test for the group_order parameter of MultiComparison. It also tests the human-readable labels in TukeyHSDResults are in the right order
Expanded docstring of MultiComparison
Added b prefix to test strings
ENH: Silence test output

A simple patch that makes minimal changes to tests that silences almost all output.
Most fixes were simply changing options to not output (e.g. disp=False), and a few
others trapped errors.

A new class was introduced in tools/testing.py that allows general output in a test
to be silenced.  It would be preferred for code to only raise warnings and not simply
print, so that the warning could be captured instead.
ENH: Converted parallel message to warning

parallel_funciton was printing a warning, but not raising one.  Converted to
a ModuleNotAvailableWarning.

Removed the 'catch all' class that was trapping this test.  Catching all text is
generally a bad idea.
ENH: Separated warnings from other code into a new file

sm_exceptions.py now holds all custom warnings and errors. ConvergenceWarning
was moved to this new module, and imports were corrected.
BUG: Correct derivative for exponential transform.
Merge pull request #1540 from jseabold/fix-1539

BUG: Correct derivative for exponential transform.
Merge pull request #1507 from bashtage/silence-tests

CLN: Silence test output
ENH: Avoid RuntimeWarning when hessian unused
TST: Remove unnecessary test.
Merge pull request #1543 from jseabold/tst-fix-warning

TST: Suppress RuntimeWarning for L-BFGS-B