plugins.xunit: Add class XmlResult.

Improvements for using xUnit  with Jenkins (Junit).
Now the actual XML result is handled in class XmlResult.
Also minor fix (cosmetics) in tests.failtest.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
Merge pull request #27 from ruda/test_result_junit

plugins.xunit: Add class XmlResult.