Style fixes in Pandora.m
Move concern of setting subscriber status to -fetchSubscriberStatus

Before code set the self.cachedSubscriberStatus twice, may have caused
a bug.
Fix login logic & Improve readability

- Add verbs to login method names
- Explicitly check if self.cachedSubscriberStatus == nil, may fix the relogin looping bug
- Add some comments and documentation
- Prefer assignment operator to setAttribute: methods
- Change PandoraRequest’s request property from MutableDictionary to Dictionary.
  This way dictionary literals can be used where “they fit in”.