ENH: percentile function with additional parameters and vecorization

The percentile function was enhanced by adding limit and interpolation
parameters to give it similar functionality to SciPy's stats.scoreatpercentile
function.  In addition the function was vecorized along q and rewritten to
use the partition method for better performance.
MAINT: cleaning up percentile function.
DOC: changes to scoreatpercentile docstring, doc test now passes
TST: additional test for percentile
DOC: improve __numpy_ufunc__ documentation

Cross-reference it from Ufunc documentation and mention the version it
was added.
Fixes in the npyio documentation

Fixes the "see also" section of savetxt, which
described savez as compressing (closes #587 ). Also
replaces all occurences of .npy and .npz to use double backticks.
Some had, some did not, and some had " symbols.
small typo
MAINT: changed 'closest' interpolation to 'nearest'
TST: note on overwrite_input parameter in percentile

* added note that `overwrite_input` has not effect when `a` is not
  an array in the percentile function.
* added unit test to verify that no error is raised when `a` is not
  an array and `overwrite_input` is True.
BUG: Fix issue with PR #3491

Don't convert unicode string to ascii string before getting byte length.
Also, get length of non string type as ascii or unicode depending on
output string type.
Merge pull request #3765 from charris/forward-port-3761

BUG: Fix issue with PR #3491
TST: exercise median of median of 5 code by using a median of 3 killer dataset
ENH: remove an always true conditional

v[1] < v[2] is guaranteed by the swapping a few lines above
Merge pull request #3658 from jjhelmus/percentile_enh_new

ENH: percentile function with additional parameters and vectorization
Merge pull request #3752 from pv/doc-some-xref

DOC: improve __numpy_ufunc__ documentation
DOC STY: make npyio docstrings less than 80 chars
BUG: make checking for sse intrinsics more robust

check for two intrinsics from [ex]mmintrin.h instead of only checking
the existance of the headers.
E.g. mingw 4.2 emmintrin.h can be included even if SSE2 is disabled.
closes #3760
Merge pull request #3753 from jarondl/fix-doc-savetxt

DOC: Fixes in the npyio documentation
ENH: Add gfortran to travis.yml configuration to enable f2py tests.
Merge pull request #3773 from charris/enable-f2py-tests

ENH: Add gfortran to travis.yml configuration to enable f2py tests.