Fixed I/O between Xliff/Xlf formats.

- Configured loader for xlf to be the same as for xliff.
- Added XliffMessage to store:
	* note-elements (children of trans-unit)
	* attribute trans-unit('approved')
	* attribute target('state').
- Added I/O passthrough of existing input, e.g. XliffMessage, in Updater.php.
- Modifications to XliffLoader and XliffDumper to use XliffMessage.
- Added tests.