Always create diff from latest Ghost we have for an object.

Also replied origin when requeueing a change with the full object.
Removed `origin` property in `Channel` since it is no longer needed.
Removed version property from `Change` and fixed tests.
Removed `Change.target` property since it is no longer needed.

Fixed some more tests.
Merge pull request #129 from Simperium/issue/128-change-dupes

Fix #128: Create Change diff against latest Ghost