Merge remote-tracking branch 'upstream/master' into iss2255

Class Extensions was moved while I was adding function. Fixed now.

Conflicts:
	src/cryptography/x509/base.py
diff --git a/src/cryptography/x509/extensions.py b/src/cryptography/x509/extensions.py
index 92a3735..74e271e 100644
--- a/src/cryptography/x509/extensions.py
+++ b/src/cryptography/x509/extensions.py
@@ -80,6 +80,13 @@
 
         raise ExtensionNotFound("No {0} extension was found".format(oid), oid)
 
+    def get_extension_for_class(self, extclass):
+        for ext in self:
+            if type(ext.value) == extclass:
+                return ext
+
+        raise ExtensionNotFound("No {0} extension was found".format(extclass), extclass)
+
     def __iter__(self):
         return iter(self._extensions)