Adding new location field (lat + long)

Started working on a location field composed of a latitude and longitude field with an auto-updated map below them, just like so : 

![Location field](https://f.cloud.github.com/assets/1606655/599897/25f7e0fa-cc5b-11e2-89d6-708254f948bb.png)

I'm not quite sure on the proper way to do this, but I added a constant variable CUZTOM_GOOGLE_API_KEY that is used to load the google map Javascript API.

Values are stored in an array, $value['lat'] and $value['long'].

I'm not sure how users are going to use this field, but I have these ideas in mind : 
- adding another field where we can type an address instead of using lat/long
- adding a "zoom" parameter, for now it's 12 if there is a marker (value) or 1 (whole world) if there is not value yet
- and maybe having an alternative to set the location by simply drag and dropping the marker on the map

I'd be curious to know what would be the best use for this field, maybe opening an issue to get some feedback would be good.

Thanks !