Separate vending and web server code

[narendasan]

Can the code for operating the machine's arm be put into a separate library, instead of fiddling with Pi GPIO in route handlers, will let us throw away one or the other if we wish

This issue was more referring to things like https://github.com/acm-uiuc/merch-embedded/blob/master/machine_controller/app.py#L23 where you are directly modifying GPIO states from a webserver. I would prefer having some library managing this and in the web server route being able to make a function call like Merch.vend("D2")