Add auto_index_callback and auto_remove_callback config option

This is a port of https://github.com/sunspot/sunspot/pull/249 which was merged into the 1-3-stable

* Auto index and auto removal can now be done in the after_commit
  callback in Rails 3, which is a better option in general because the
  transaction will have been finished.
* Since switching to after_commit might be a breaking change for some
  (especially in tests), it was decided to add a config option that
  defaults to after_save.