Pregenerate bootable RPi .imgs in the cloud, from recipes (Public Lab)

^ based on Hypriot project, each pull request against our repository generates a prebuilt SD card image based on vanilla Raspbian: https://github.com/publiclab/pi-builder/pulls/

Reproducible SD card images: downloads and boots Raspbian, runs recipe, re-generates .img file in about 10m. Recipes welcome!