switch between the sandbox and real amt

Though I cannot access to the real amt, I have tested that the switch worked when I want to use the sandbox. 
Ah, good point. The README.md file doesn't document post.py fully at all. I think I'm ok with that, as long as running `python post.py --help` is informative enough to use the whole script. But you could add a 'Posting example tasks to the server' section that documents all of post.py's functionality if you'd like.