Add ability to move a task's position within the list

Like the subject states, I added the ability to move a task's position within the task list. Also added some specs to help in developing the change. I think these may be useful to pull in.

I'd also like to add some other features relatively soon ... tags or categories or projects, not sure what to call it. I usually title a pomodoro with "Project: Title of pomodoro", and it'd be nice to keep track of a project so that you can list only pomodoros in a certain project for example.

Let me know what you think about that when you have the time.

And of course, thanks for writing this great little tool!