Added support for scheduling blocks to be called when the timer fires

After I stumbled upon this great library via CocoaPods, I've realized that it would be much easier for me to use the timer via "scheduling" a block. I know that there's dispatch_after, but I really want a clean way to cancel the scheduled block (actually cancel, not make it just skip everything when it's eventually fired anyway). So I've added some more code into the class, trying to keep it analogous to the existing one.
If you think these changes will do good to the library, feel free to merge :)

Oh, and I've also fixed a couple of typos in comments.