Adding command line in xunit plugin to specify test suite name

Hi guys, 

I opened a bug #859, as right now there is no way to specify test suite in xunit plugin as .  I have added one more command line paramter (--test-suite-name) through which you can specify the test suite name as well. 

adding this pull request to resolve #859
Running things locally and using `-x` to stop at the first failure, I can see that you've broken the formatting by adding in the addition here.  You should probably take another approach, like:

```python
stats_and_name = self.stats.copy()
stats_and_name['test_suite_name'] = self.test_suite_name
        self.error_report_file.write(
            u'<?xml version="1.0" encoding="%(encoding)s"?>'
            u'<testsuite name="%(test_suite_name)s" tests="%(total)d" '
            u'errors="%(errors)d" failures="%(failures)d" '
            u'skip="%(skipped)d">' % stats_and_name)
```

Note: I haven't tested the above code at all.