Added a simple XML generator tool

After having written a script for genearating XML files for tenth or so
time decided to add one in git as well.
Publish session after save, and then re-save

If test cases contain measurements we need to save them separately
instead of using activerecord-import. This will trigger test session
export observer on every save which in turn makes ActiveRecord load the
whole session from database.

By first creating and saving the session, and then publishing and saving
it again, export trigger is actually executed only once which bring
significant performance improvements when run with e.g. 100 cases, each
with a single measurement:

Original implementation: throughput time 6.7 seconds
New implementation: throughput time 2.1 seconds