Implemented TestNG-compatible XML formatter.

Hi there!
This class produces TestNG-compatible XML report, including date\time, duration and stacktraces if possible.
I used JUnitFormatter as an example with some modifications and simplifications.
All tests call formatter directly without using CucumberRuntime.
What about wrapping Exceptions with Mockito, latter does not allow wraping of getClass method so I had to use the real Exception object...