Everything but the first sentence is indeed by King and can be found in Strength to Love. That first sentence, though, is a complete fake.
...
Retractions and corrections on real-time social services like Twitter are nearly impossible. As is so often the case, the great Internet fact-checking machine is already in full swing, with discussions on Reddit and numerous blogs. That, however, will do little to reach all of those who retweeted this fake quote today. Indeed, this fake quote will likely become part of the MLK Jr. canon soon. While many will post about how this quote is fake, these stories will only reach a minority of those who read it today. Instead, it’s still being retweeted a few times per minute and continues to appear on new blog posts and Facebook status updates.
