Merge pull request #268 from phvannor/offline

Offline
[client][managed][offline] Preserve system columns on local store
Merge pull request #269 from phvannor/offline

iOS Clean up documentation, update local store
[client][managed][offline] improve performance of bulk insert and delete
[client][managed][offline] support for soft delete
1442151 [client][managed][offline] GetSyncTable should throw if sync context is not initialized
1542091 [client][managed][offline] on pull do not upsert items without an id
1464259 [client][managed][offline] PullAsync should request system properties
1442190 [client][managed][offline] NullReferenceException if accessing MobileServiceClient.SyncContext.PendingOperations before initializing the sync context
Update error logic process for failed push op

Finish condense todo item, clean up some documentation on core data
store, and extra test cases