Do not connect to the SWF API when instanciating models

All models inherit from [`core.ConnectedSWFObject`](https://github.com/botify-labs/python-simple-workflow/blob/master/swf/core.py#L16) that connects to the SWF API in the constructor. It requires AWS credentials anywhere we instanciate models even if they don't query the API.

The connection should be lazy and happen only when needed.
