Reindexer shouldn't fail when target exists

It's a common use case to reindex without creating the target. For
instance, after reindexing, an operator may cut over new writes and
then reindex again to ensure any writes that ocurred during the
reindex are replayed to the new index.
PEP8
Make reindex.py executable
Releasing v0.13.1
Bump version to 0.14.0-dev