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