Hi

I have issues creating sns subscriptions through Cloudformation. If I attempt to include a `AWS::SNS::Subscription` resource in a Cloudformation template and run `create-stack` then I get a 500 Internal Server error. The full template:

```
AWSTemplateFormatVersion: '2010-09-09'
Description: 'Test'
Resources:
  MySnsTopic:
    Type: AWS::SNS::Topic
    Properties:
      TopicName: MySnsTopic
  MySnsTopicSubscription:
    Type: AWS::SNS::Subscription
    Properties:
      Protocol: sqs
      TopicArn: !Ref MySnsTopic
      Endpoint: !GetAtt
        - MySqsQueue
        - QueueArn
  MySqsQueue:
    Type: AWS::SQS::Queue
    Properties:
      QueueName: MySqsQueue
```

If I replace the references with hard-coded arns like this:

```
TopicArn: arn:aws:sns:eu-west-2:123456789012:MySnsTopic
Endpoint: arn:aws:sqs:elasticmq:000000000000:MySqsQueue
```

Then I get no errors but I get a warning:

`No moto Cloudformation support for AWS::SNS::Subscription`.

I can get around this issue by creating the subscription when creating the topic but the issue there is that I cannot add any attributes and in my case I need `RawMessageDelivery` set to true. So my questions are:

1. Is there no support to create `AWS::SNS::Subscription` resources through Cloudformation or am I doing something wrong?
2. If support is not there can I request it as a new feature?
3. Any suggestions on how I can work around this? I know I could do this through a script at startup but would like to not have any additional script steps other than deploying the Cloudformation stack.

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pass
def make_http_request(url, data=None, headers=None, method='GET'):
    if is_string(method):
        method = requests.__dict__[method.lower()]
    return method(url, headers=headers, data=data, auth=NetrcBypassAuth(), verify=False)
class SafeStringIO(io.StringIO):
    """ Safe StringIO implementation that doesn't fail if str is passed in Python 2. """
    def write(self, obj):
        if six.PY2 and isinstance(obj, str):
            obj = obj.decode('unicode-escape')
        return super(SafeStringIO, self).write(obj)
def clean_cache(file_pattern=CACHE_FILE_PATTERN,
        last_clean_time=last_cache_clean_time, max_age=CACHE_MAX_AGE):
    mutex_clean.acquire()
    time_now = now()
    try:
        if last_clean_time['time'] > time_now - CACHE_CLEAN_TIMEOUT:
            return
        for cache_file in set(glob.glob(file_pattern)):
            mod_time = os.path.getmtime(cache_file)
            if time_now > mod_time + max_age:
                rm_rf(cache_file)
        last_clean_time['time'] = time_now
    finally:
        mutex_clean.release()
    return time_now
def truncate(data, max_length=100):