several major changes and improvements to reachability handling

- added an api request timer to periodically update user data and avatar
- deauthorising if first api request fails (no local data to load)
- public repo timer is no longer triggered by a successful api request but started independently after starting the api timer
- menu icon is adjusted on startup if unreachable
- refactored variables used during status request (in -pollGithub) to differentiate from actual api request variables
- better http request error handling and debug output (silently ignoring NSURLErrorNotConnectedToInternet failures, but responding to 401 and 403 errors likely to signal access revocation)
- timers are invalidated when unreachable, and changes to status timer interval ignored (user defaults is set, but new value is unused until reachable again)
- status menu icon changes depending on reachable state
- local user data (api request json and user avatar) are removed when deauthorising