Rewrite the PasswordStoring class

PasswordStoring class has been rewritten to not be tied to the Mac version of the Radar settings UI. QRRadarSubmissionService class now uses the new method radarPasswordForAccount:error: from the PasswordStoring class to retrieve the Radar password.

In QRRadarSubmissionServicePreferencesViewController I wasn't sure what the best way would be to display an error if one occurs while setting a new password in the Keychain, so it just displays the error in the console.