Added basic support for SQS MessageAttributes.

Also if you really want to compare types use `isinstance()` but what you really want is to coerce to tuple if the variable is not a string and catch the type error and handle accordingly.

```python
try:
  message_attributes = tuple(message_attributes)
  return message_attributes[0], message_attributes[1]
except TypeError: 
  message.message_attributes = message_attributes
  # ...
```