Implement N806, 815, 816, 818 (#501)

resources/test/fixtures/N806.py

@@ -0,0 +1,4 @@
+def f():
+    lower = 0
+    Camel = 0
+    CONSTANT = 0

resources/test/fixtures/N815.py

@@ -0,0 +1,6 @@
+class C:
+    lower = 0
+    CONSTANT = 0
+    mixedCase = 0
+    _mixedCase = 0
+    mixed_Case = 0

resources/test/fixtures/N816.py

@@ -0,0 +1,5 @@
+lower = 0
+CONSTANT = 0
+mixedCase = 0
+_mixedCase = 0
+mixed_Case = 0

resources/test/fixtures/N818.py

@@ -0,0 +1,10 @@
+class Error(Exception):
+    pass
+
+
+class AnotherError(Exception):
+    pass
+
+
+class C(Exception):
+    pass