Issue #52. New method 'rollLogFileWithCompletionBlock:' in DDFileLogger. 'rollLogFile' method is deprecated now.

Completion block is optional and will be called on main queue.
Issue #55. DDFileLogger: monitoring current log file. If file is moved/removed - rolling it and creating new one.
Issue #129. DDFileLogger: on iOS, when creating file now we are setting appropriate NSFileProtectionKey

(NSFileProtectionCompleteUntilFirstUserAuthentication if app is able launch from background and NSFileProtectionCompleteUnlessOpen in other cases).