Remove whitespace errors
Prevent multiple concurrent dirty/destroyed sync calls

If syncDirtyAndDestroyed gets called again before a success callback gets executed,
multiple create requests will be sent to the server for each dirty new record,
even for updates. This commit prevents this from happening.

Throttle allows at most 1 request to be queued behind a pending request.
Newly added requests replace any existing queued request.

Closes #35