add resultclass for runner instanciation

... as described in unittest's doc ...
_makeResult can be overridden in subclasses or you can control
the resultclass provided to the runner. This may help separate
concerns w.r.t. integration with other pieces of code that
want to hook into the test result reporting
(e.g. pull request #98)

xmlrunner/runner.py

@@ -16,7 +16,8 @@ class XMLTestRunner(TextTestRunner):
     """
     def __init__(self, output='.', outsuffix=None, stream=sys.stderr,
                  descriptions=True, verbosity=1, elapsed_times=True,
-                 failfast=False, buffer=False, encoding=UTF8):
+                 failfast=False, buffer=False, encoding=UTF8,
+                 resultclass=None):
         TextTestRunner.__init__(self, stream, descriptions, verbosity,
                                 failfast=failfast, buffer=buffer)
         self.verbosity = verbosity
@@ -28,13 +29,18 @@ def __init__(self, output='.', outsuffix=None, stream=sys.stderr,
             outsuffix = time.strftime("%Y%m%d%H%M%S")
         self.outsuffix = outsuffix
         self.elapsed_times = elapsed_times
+        if resultclass is None:
+            self.resultclass = _XMLTestResult
+        else:
+            self.resultclass = resultclass
 
     def _make_result(self):
         """
         Creates a TestResult object which will be used to store
         information about the executed tests.
         """
-        return _XMLTestResult(
+        # override in subclasses if necessary.
+        return self.resultclass(
             self.stream, self.descriptions, self.verbosity, self.elapsed_times
         )