Return DkimTokens as a list for SES identity calls

Applies the fix in #1550 from @cellofellow and adds a unit test to ensure the fix works. See the original issue for a full explanation of the problem.

Before the fix:

```bash
$ python test.py unit/ses
nose command: test.py -a !notdefault unit/ses
F
======================================================================
FAIL: test_ses_get_identity_dkim_list (tests.unit.ses.test_identity.TestSESIdentity)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/taylordt/Projects/Amazon/boto/tests/unit/ses/test_identity.py", line 69, in test_ses_get_identity_dkim_list
    self.assertEqual(ListElement, type(tokens))
AssertionError: <class 'boto.jsonresponse.ListElement'> != <class 'boto.jsonresponse.Element'>
-------------------- >> begin captured logging << --------------------
boto: DEBUG: Using access key provided by client.
boto: DEBUG: Using secret key provided by client.
boto: DEBUG: Method: POST
boto: DEBUG: Path: /
boto: DEBUG: Data: Action=GetIdentityDkimAttributes&Identities.member.1=test%40amazon.com
boto: DEBUG: Headers: {'Content-Type': 'application/x-www-form-urlencoded; charset=UTF-8'}
boto: DEBUG: Host: email.us-east-1.amazonaws.com
boto: DEBUG: Params: {}
boto: DEBUG: establishing HTTPS connection: host=email.us-east-1.amazonaws.com, kwargs={'timeout': 70}
boto: DEBUG: Token: None
--------------------- >> end captured logging << ---------------------

----------------------------------------------------------------------
Ran 1 test in 0.006s

FAILED (failures=1)
```

After applying the patch:

```bash
$ python test.py -v unit/ses
nose command: test.py -a !notdefault -v unit/ses
test_ses_get_identity_dkim_list (tests.unit.ses.test_identity.TestSESIdentity) ... ok

----------------------------------------------------------------------
Ran 1 test in 0.006s

OK
```