MAINT: clean up comments/commented-out code in distributions.py; some PEP8 fixes.
MAINT: enable rdist._cdf, making it a lot faster.  Closes gh-1285.

Issue for hyp2f1 that is worked around is discussed in gh-1561.
ENH: add central moment methods to invweibull and loglaplace distributions.

Faster, more accurate and removes a warning generated by the distribution fit
test for invweibull.
TST: speed up stats tests some more, and add run_module_suite to each test file
Merge pull request #2612 from rgommers/stats-distributions-cleanup

MAINT: clean up comments/commented-out code in distributions.py
[stats] Allow for keyword arguments in distributions.

Generates the correct signatures for args, loc, scale
dynamically for each distribution. Thus sidesteps the
using _fix_loc_scale_kwarg3 at each call.
Generates signatures for stats and moments as well,
allowing `size` and `moments` as keywords or
positional arguments.
TST: Change the test allowing propagation of nans with
incorrect number of parameters. The changed behavior matches
that of R.
TST: test all distributions
TST: refactor tests, remove unreacheable code
MAINT: some minor edits to PR-2588, style and update gh-issue number.