Started API

# Why?

 * As Chase and Simon brought up, we really ought to have an API.

# What's New?

 * Base scafolding of a JSON API. 
 * Only had time for crop#list today, but feel free to check it out at `api/crops` or `api/crops.json`

Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.