URLs count towards the 140-character limit in their t.co shortened form

Use twitter-text-objc [1] to calculate the lengths of status messages when determining whether to truncate them or not. This library correctly calculates URLs in their shortened format, as Twitter does according to their FAQ [2].

[1] https://github.com/twitter/twitter-text-objc
[2] https://dev.twitter.com/docs/faq