Calculate the time of arrival for buses in the API

[wbprice]

Kevin Curry and Kevin Griffin have reported that the front-end will deliver different time-to-arrival estimations for different users standing next to each other. To get around this, please create an estimated time of arrival in the API.

The logic for finding the time is described here:
https://github.com/wbprice/hrt-bus-finder/blob/gh-pages/js/busfinder.js#L5-L21

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