ENH: stats: Rewrote stats.obrientransform:

* Vectorized and simplified the computation.  For largeish arrays,
  this gives up to 60 times speed improvement.
* Copy-edited the docstring, and added 'References' and 'Examples'.
* `TINY` changed from 1e-10 to `np.sqrt(np.finfo(float).eps)`.
* Added another test.
BUG: stats: make sure obrientransform doesn't crash using numpy 1.5.x when the shapes of the arguments are not all the same
DOC: update 0.13.0 release notes.
DOC: Add scipy.sparse release notes.
DOC: add authors list to 0.13.0 release notes.

[ci skip]
DOC: stats: Improve the example in the obrientransform docstring to show passing the results to f_oneway
BUG: Error for matrices too big for Matlab 5

Matlab 5 format can only hold matrices < 2**32 bytes in size.

Give error message when saving for matrices >= 2**32
Merge pull request #2631 from WarrenWeckesser/stats-obrientransform

ENH: stats: Rewrote stats.obrientransform:
Merge pull request #2710 from matthew-brett/big-matrix-fix

BUG: Error for matrices too big for Matlab 5
DOC: Clarify that step's system parameter is a tuple of array_likes

also reorder parameter docs to match function's ordering
BUG: Docstring claims to accept array_like for T, but does not

Tries to call T.shape, but this produces

"AttributeError: 'list' object has no attribute 'shape'" etc.

change so that it explicitly converts to array
MAINT: Remove U = zeros_like(T), since that's already lsim2's default when U=None
DOC: Remove a comment redundant with nearby docstring,
change a comment to docstring,
spell check a comment, add TODO to a comment
TST:Create tests for step and impulse based on tests for step2 and impulse2

Didn't copy tests that pass for step2 and impulse 2 but fail for step and
impulse, though these should probably be addressed at some point
TST: Add tests for step/impulse/step2/impulse2 for accepting array_like without errors
BUG: fix a couple of bugs in signal.ltisys
TST: de-duplicate tests for signal.ltisys
TST: add regression test with complex input for signal.step.

Note that step2/impulse/impulse2 don't work with this input (see gh-2654).

When unifying step/step2 and impulse/impulse2, a decision should be made on
whether complex inputs need to be supported.
Merge branch 'pr/2654' into master.

See https://github.com/scipy/scipy/pull/2654
Merge pull request #2685 from rgommers/update-release-notes

DOC: update 0.13.0 release notes.