a once and stop file for halting

Checks for usync.once on read...
Fixe for PreValue order

Mimiced DataType ToXML so we can control the order prevalues are saved.
1.2.1 - PreValue fix/fix
Merge remote-tracking branch 'upstream/master'

Conflicts:
	jumps.umbraco.usync/SyncDataType.cs
	jumps.umbraco.usync/SyncDocType.cs
	jumps.umbraco.usync/uSync.cs