ENH: add Series.str iterator
closes #3638.

release notes!
 ah crap my bad i always forget about RELEASE.rst :s
 done
 oh bollocks `assertIsInstance` is not a method in 2.6 :(
 @jreback this can be merged. quite a few warnings in `RELEASE.rst` about unknown targets. want me to fix those up?
 are these about incorrect gh numbers that don't map to issues? (e.g. are wrong)?

maybe do a separate pr for that
 issues r not wrong they r just not mapped to a link, e.g., `GH3504_` doesn't have the corresponding `.. _GH3504 https://github.com/pydata/pandas/issues/3504`.
 further motivation for me 2 write the darn sphinx ext
 ne way i will clean up `RELEASE.rst`, i.e., get rid of non spurious warnings. hope this will not cause 2 much git merge conflict headaches...
 merged..thanks!
