Enable default JSON response at API root.

This replaces the current "invalid request" message at the root of the API site (e.g., http://devapi.opentreeoflife.org or http://api.opentreeoflife.org/) with the intended response, a tiny JSON document that points to API docs and source code.

This is available for review on **devapi**. I've tested with a few method URLs from v1 and v2 APIs, and it seems to leave all other URLs unaffected.