Vend from queue instead of by item

[narendasan]

I should be able to select as many items as I want and checkout. Then the machine should just vend them all continuously (instead of vend one, then select another, the wait for that to vend)

Basically vend from a queue instead of waiting for each item individually (see acm-uiuc/merch-ios#1)

[abrandemuehl]

Based on the discussion on #6, I'm going to close this issue