TODO

- length restrictions for atts
- Paginated results and iterators
- Waiters (methods that wait for an object to be in a determined state)
- Retrying
- Exception handling
- JSON callers
  - when a boolean is returned from API, it doesn't get properly handed to 
    result objects (error thrown)
- rest-xml callers
- enums for results may be too strict. If AWS returns something new, an exception
  is raised
- Pass flattened lists and unflattened lists correctly
- Some APIs (EC2 at least), will have different definitions for same shapename when its
  input vs output. Outputs will specify xmlname, and inputs won't.
- Use Metadata service to get access_key, secret_key and token from it if no creds are
  specified