BUG: ensure percentile has same output structure as in 1.8

percentile returned scalars and lists of arrays in 1.8
adapt new percentile to return scalar and arrays with q dimension first
for compatibility.
DOC: fix signature of NpyIter_GetIterNext in capi doc
DOC: add highlights to release notes and rewrite and move some sections

closes gh-3842, gh-3843
Merge pull request #3868 from charris/forwardport-1.9-3859

Forwardport 1.9 3859
Merge pull request #3769 from juliantaylor/percentile-compat

BUG: ensure percentile has same output structure as in 1.8