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
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.
Merge pull request #3658 from jjhelmus/percentile_enh_new

ENH: percentile function with additional parameters and vectorization