return a cancellable object, now we can stop streaming with OS accounts, fix https://github.com/nst/STTwitter/issues/81
added missing class STTwitterOSRequest into project
fix https://github.com/nst/STTwitter/issues/78 and use a more explicit error domain for Twitter errors: "STTwitterTwitterErrorDomain"
added an NSError category to convert an HTTP response from Twitter API into an NSError
refactored error handling, now provide rate limits inside in the userInfo, fix https://github.com/nst/STTwitter/issues/79
added reference to twitter-text-objc