Add GeoJSON documentation (Closes LOGSTASH-1703)

Also:
 * Set the 'source' option to required, as the plugin is useless without
it.
 * Add the 'location' field to the rspec
Merge pull request #874 from pieterlexis/LOGSTASH-1703

Documentation changes to GeoIP regarding GeoJSON, geo_point, and the new location field.

Thanks for making Logstash more awesome!