Support array geo-points. (Was: Map widget shows error "Map widget is only available for fields containing geo data.")

### Problem description

Messages are transmitted via Logstash and the following config:

```
if [clientip] {
geoip {
source => "clientip"
target => "geoip"
}
}
```

A Graylog message will include:

```
clientip: 197.97.73.146
geoip_continent_code: AF
geoip_country_code2: ZA
geoip_country_code3: ZAF
geoip_country_name: South Africa
geoip_ip: 197.97.73.146
geoip_latitude: -29
geoip_location: [24.0,-29.0]
geoip_longitude: 24
geoip_timezone: Africa/Johannesburg
```

When clicking on the "World Map" option under the field "geoip_location" the following error is displayed:

```
Could not load map information
Map widget is only available for fields containing geo data.
```

It would also be good to remove "World Map" from fields where it is impossible to show a world-map (i.e. fields not being IPs or fields not containing coordinates.
### Steps to reproduce the problem
1. See above
### Environment
- Graylog Version: Graylog 2.0.0-beta.3 (1d03037)