Add better logic for loading entry points

As noted in the code, setuptools 11.3 deprecated EntryPoint.load and
its require parameter. Starting with 11.3 the preferred way is use
EntryPoint.require and EntryPoint.resolve as necessary. Unfortunately,
those methods do not exist in all versions of setuptools. So we have
to check for them and use them when they're available. Otherwise, we
fallback to using EntryPoint.load with the require parameter.

Closes #59
Closes #90

flake8/tests/test_engine.py

@@ -113,6 +113,30 @@ class TestEngine(unittest.TestCase):
             sg = engine.StyleGuide(parser=parser)
             assert 'X' not in sg.options.ignore
 
+    def test_load_entry_point_verifies_requirements(self):
+        entry_point = mock.Mock(spec=['require', 'resolve', 'load'])
+
+        engine._load_entry_point(entry_point, verify_requirements=True)
+        entry_point.require.assert_called_once_with()
+        entry_point.resolve.assert_called_once_with()
+
+    def test_load_entry_point_does_not_verify_requirements(self):
+        entry_point = mock.Mock(spec=['require', 'resolve', 'load'])
+
+        engine._load_entry_point(entry_point, verify_requirements=False)
+        self.assertFalse(entry_point.require.called)
+        entry_point.resolve.assert_called_once_with()
+
+    def test_load_entry_point_passes_require_argument_to_load(self):
+        entry_point = mock.Mock(spec=['load'])
+
+        engine._load_entry_point(entry_point, verify_requirements=True)
+        entry_point.load.assert_called_once_with(require=True)
+        entry_point.reset_mock()
+
+        engine._load_entry_point(entry_point, verify_requirements=False)
+        entry_point.load.assert_called_once_with(require=False)
+
 
 def oserror_generator(error_number, message='Ominous OSError message'):
     def oserror_side_effect(*args, **kwargs):