ENH: add StringMethods (e.g. .str) to Index

currently

`Series.str.strip()` works
but 

`Index(Series(Index.values).str.strip())` would be necessary for an index

should be very straightforward to move the `.str` accessor to `IndexOpsMixin` (from Series)
to support this (would need to prevent Index sub-classes from using it though as they don't support string ops.
