Clean up and reorganization of Pandora.m

- #pragma mark organization
- Rename terse method names in favor of more verbose message passing
- Add extra documentation to important properties
Implement NSCopying on PandoraRequest
Rename +errorString: to +stringForErrorCode
Rename -notify:with: to -sendNotification:withUserInfo:
Re-log-in with Pandora One Partner Login if user is a Pandora One subscriber