ENH: support StringMethods index and rindex
as a part of https://github.com/pydata/pandas/issues/9111

@sinhrks @jreback 

are there any other routines that have an `array` as a return type?
 @jreback good catch, return type of `array` is used in the old docstrings, I'll update it

I also added to the release note 
 @jreback do you think this can make 0.16.1? seems like it'd be nice to get all the string methods in
 @jreback i updated the release note

the only other PR remaining is for `str.translate()` here: https://github.com/pydata/pandas/pull/10052/ 
 was also passing before, right?
 yes this was passing too, just changing docs 
