XADataSource implementation

Hi,

I've been working on the XADataSource implementation. Patch attached. Test case
is in the separate zip file, because I couldn't get cvs to include it in the
diff.

I looked at Micheal Allman's implementation, but decided to continue working on
my original version instead. Michael's implementation was perhaps more
ambitious, it had more thorough bookkeeping for resource associations and xids.
Unfortunately there's no straightforward way to implement the full XA interface
in a spec-compliant way. I did copy some snippets and test cases from Michael's
code that were useful, though.

Thanks Michael for your hard work, it was a lot of help anyway!

This implementation doesn't try to support transaction interleaving as required
by the XA spec. Instead, it throws an exception saying it's not supported.

I tested it with the JOnAS application server (after fixing some recovery bugs
in JOnAS itself :), and it seems to be fine with the limitations. It needs
testing with other application servers to see if the limitations matter for
them.

Anyone with access to WebLogic, WebSphere etc: please help! I can send the
simple servlet I used to test JOnAS if you're interested.

BTW: How does the translation work? What strings need to be translated?

- Heikki