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. 
This is awkward, since the data is different for each crowd. Perhaps a better solution would be to have the `send_request()` function take just take the base data, and then instead of a list of crowd names, a dictionary from crowd names to additional configuration data (e.g., `{'amt': {'sandbox': 1}, 'internal': {...} }`).