Bug fix: Plugin Sync
1. Failed to create more than one realtime document.
   Pass assignIdCallback and failedCallback as function parameter for FileSearcher instead of saving it.

2. If a client with empty data use Sync plugin first, other clients will lose their data once connected with Realtime API.
   Assign '' to lastUpdateUUID on model initialization if the client do not have data.

3. Realtime API sometimes return 'CLIENT_ERROR' instead of 'NOT_FOUND' if a file is not found
   Change handling of 'CLIENT_ERROR' to the same as 'NOT_FOUND'
Plugin Sync: bump version number to 0.2.1
Merge pull request #509 from Xelio/patch-plugin-sync-5

Bug fix: Plugin Sync