Add a get_session() utility function

[jesper-friis]

Add a get_session() utility function and replace all current occurences of

AttrDict(**json.loads(raw_session))

with get_session(raw_session), where raw_session is what is returned by the fetch(), get() and initialize() methods.

This makes the code easier to read and makes it much easier to change the return value of the fetch(), get() and initialize() methods as suggested in issue #86.

If issue #86 is rejected, this utility function would still be improve the user-friendliness (although not as much as issue #86).

[CasperWA]

I don't think this is relevant any longer? Issue #86 is being implemented by #134.