Fix order of JMS custom elements as per XLIFF spec

Sending a PR on behalf of @ChMat for issue #39

As per OASIS XLIFF 1.2 specification document (http://docs.oasis-open.org/xliff/v1.2/os/xliff-core.html#trans-unit), this is the order to follow in trans-unit elements:
Contents:
One <source> element, followed by 
Zero or one <seg-source> element, followed by 
Zero or one <target> element, followed by
Zero, one or more <context-group>, <count-group>, <prop-group>, <note>, <alt-trans> elements, in any order, followed by
Zero, one or more non-XLIFF elements.
All child elements of <trans-unit> pertain to their sibling <source> element.
While for backward compatibility reasons no order is enforced for the elements before the non-XLIFF elements, the recommended order is the one in which they are listed here.