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

[jreback]

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.