ENH: Implement Series.StringMethod.slice_replace
closes #8888 

This does not implement immerrr's suggestion of implementing `__setitem__`, as all `StringMethod` methods return a new `Series` (or `DataFrame`) rather than mutating. 

This looks pretty good to me. We are about to release 0.15.1, so after we do that please add release notes for this new feature.
 Sure, I'll add a release note and rebase after the release drops.
 @billletson 0.15.2 has been released, so you can go ahead and add release notes now for 0.16.0.
 @billletson sorry, I was not paying attention earlier. Could you please rebase again and ping me when the tests pass so I can do the merge? Github does not send out notifications of new commits without a comment :/.
 @shoyer rebased and build passing, good to go
 Thanks!
 @billletson this is nice.

maybe stick an example in here somewhere: http://pandas-docs.github.io/pandas-docs-travis/text.html ?
