Simperium Mk 0.6.8

### Release Details:

- Issue #332: Invalid remote changes were hitting an internal success callback.
- Issue #334: Implemented safety measures to ensure that objects are faulted while processing.
- Issue #335: Added several failsafes to SPIndexProcessor.
- Issue #337: Legacy locally-generated changesets are nuked + recalculated after a rebase OP.
- Issue #339: Renames an internal NSError category.