This commit adds a third parameter to the L.LatLon class for specifying
altitude. This is in turn stored in the `.latitude` property for the LatLng
instance. Latitude property will only be set if the latitude parameter is not
undefined, this is done in order to ensure backwards compability.
```javascript
var latlng = new L.LatLng(10, 20, 30);
console.log(latlng.altitude); // prints '30' to the console
```
Similar functionality has been added to L.GeoJSON coordsToLatLng() and
latLngToCoords() methods in order to handle import and export of 3D GeoJSON.
```javascript
var geoJSON = {
type: 'Feature'
,properties: {}
,geometry: {
type: 'Point'
,coordinates: [20, 10, 30]
}
}
var layer = new L.GeoJSON();
layer.addData(geoJSON);
console.log(layer.getLayers()[0].getLatLng().altitude);
```
`NB` It is important to notice that no logic has been added in order to prevent
latitude and longitude to change without appropirate change in altitude – this
must be handled by the application.
