[100] XA transaction support

This PR is for support multiple ceylon.dbc datasources within a single transaction.

I created a directory for examples under  ceylon-sdk/source/example (gavin suggested ceylon-sdk/test-example but then the ceylon compile and run tools would not know where to find the sources). The example shows recovery which is difficult to do in the standard tests (test-source/test/ceylon/transaction/).

Stephane, I am sorry that I lost your commits on my branch (I lost them in the process of creating a clean commit history - sorry).