Change observe-sequence callbacks

We now call 'removedAt' and 'changedAt' with indices (instead
of 'removed' and 'changed'). While at it, fix a bug where
the 'movedTo' callback was firing with incorrect
indices.

commit f44d03d2c8bbfca32ae08fdabef35472c62042eb
Author: Avital Oliver <avital@thewe.net>
Date:   Tue Apr 15 14:54:29 2014 -0700

    A more complicated movedTo test that should catch a bug

commit 24f9ba3ffc708cf1af34e434ddf033c54d246614
Author: Avital Oliver <avital@thewe.net>
Date:   Tue Apr 15 14:05:44 2014 -0700

    Update tests and {{#each}} for new observe-sequence