Index coordinates as geopoints in geonames track

With this commit, we store locations not as latitude / longitude double
values but as `geo_point` instead. We also use the most recent version
of the geonames dataset and enhance documentation a bit.

Closes elastic#2

geonames/README.md

@@ -0,0 +1,20 @@
+## Geonames track
+
+This track is based on a [geonames](http://www.geonames.org/) dump of the file [allCountries.zip](http://download.geonames.org/export/dump/allCountries.zip) retrieved as of April 27, 2017. 
+
+For further details about the semantics of individual fields, please see the [geonames dump README](http://download.geonames.org/export/dump/readme.txt).
+
+Modifications:
+
+* The original CSV data have been converted to JSON.
+* We combine the original `longitude` and `latitude` fields to a new `location` field of type [geo_point](https://www.elastic.co/guide/en/elasticsearch/reference/current/geo-point.html).
+
+### License
+
+We use the same license for the data as the original data from Geonames:
+
+```
+This work is licensed under a Creative Commons Attribution 3.0 License,
+see http://creativecommons.org/licenses/by/3.0/
+The Data is provided "as is" without warranty or any representation of accuracy, timeliness or completeness.
+```

geonames/mappings.json

@@ -24,8 +24,8 @@
           }
         }
       },
-      "longitude": {
-        "type": "double"
+      "location": {
+        "type": "geo_point"
       },
       "cc2": {
         "type": "text",
@@ -92,9 +92,6 @@
           }
         }
       },
-      "latitude": {
-        "type": "double"
-      },
       "feature_code": {
         "type": "text",
         "fields": {