Fix #859 Xunit plug-in not able to change testsuite name

Added command line parameter in Xunit plug-in to specify test suite
name. Also added functional test case.
One too many blank lines.  Also, I'd rather the parens were cuddled with the end of the help:

```python
       parser.add_option(
           '--xunit-testsuite-name', action='store',
           dest='xunit_testsuite_name', metavar="PACKAGE",
           default=env.get('NOSE_XUNIT_TESTSUITE_NAME', 'nosetests'),
           help=("Name of the testsuite in the xunit xml, generated by plugin. "
                 "Default test suite name is nosetests."))
```