Update allowed list of layers for Geonames

[orangejulius]

The list of allowed layers for Geonames is out of date. Over time we've added better layer mappings to the Geonames importer, but haven't kept the API list up to date.

We need to ensure that every layer that a Geonames record can be is listed in the API. I think we have to add macroregion and localadmin, but there may be more.

Compare these two queries to see the issue. The first one searches Geonames for Detroit, and the second result is of type localadmin. The second query searches Geonames for only localadmin records, but it returns an error saying no such records exist (they obviously do)

http://pelias.github.io/compare/#/v1/search%3Fsources=gn&size=2&text=detroit,%20mi

http://pelias.github.io/compare/#/v1/search%3Fsources=gn&layers=localadmin&size=2&text=detroit,%20mi

Related to #484 (and could probably be done at the same time)

[orangejulius]

Oh also, as in #484, we need to ensure the documentation is up to date.

[orangejulius]

Acceptance tests would be good to add as well.