Prevent attribute access errors for unauthenticated API calls.

For methods that require OAuth credentials that were not set, we shouldn't fail
with an AttributeError; there are proper exceptions for that that are thrown
in .
Remove unnecessary authentication requirements for some API calls.

* get_object_from_short_code does not require authentication, according to rdio API docs.
* get_object_from_url does not require authentication, according to rdio API docs.