create backup and restore for 'timecreated' field in user table

Recently we are adding a new field called 'timecreated' to user table. (MDL-21226).

As suggested by Eloy, the additional field needs to be also handled by backup and restore.

It should achieve:
1) On backup, save the timecreated field to XML
2) On restore, load the timecreated field from XML
3) On restore, if new user is being created, set it to time()