New statement "map_to_area" for area in area queries

`Map_to_area` statement's objective is to convert all ways/relations in the input set to their corresponding area ids (assuming they exist, otherwise they will be silently discarded).

It can be used for area in area queries, which are not possible as of now (see issue #45) - or - require rather [complicated workarounds](http://wiki.openstreetmap.org/wiki/DE:Overpass_API/Beispielsammlung#Kreise_ohne_fire_station_anzeigen). In a way, it works like the pivot filter, just in the opposite direction.

A more direct approach like `area(area.b)` would of course be also very nice, but I found this quite difficult to implement. On the other hand converting ways/relations to areas turned out to be really straight forward.

Coding was tested using the following examples on a **local Overpass API instance**

**Find all Pubs in the Kölner Innenstadt**

    area[name="Köln"]->.b;
    rel(area.b)[name="Innenstadt"];
    map_to_area -> .a;
    node(area.a)[amenity=pub];
    out meta;


http://overpass-turbo.eu/s/56N

![koln](https://cloud.githubusercontent.com/assets/5842757/4344270/f3efc592-4084-11e4-9b68-ee48b81b86c2.png)


**Find all counties in Hessen without fire station**

    area[admin_level=4]["name"="Hessen"][boundary=administrative]->.boundaryarea;
    
    ( node(area.boundaryarea)["amenity"="fire_station"];
      way(area.boundaryarea)["amenity"="fire_station"];>;) ->.a;
    
    .a is_in -> .b; 
    area.b[admin_level=8] -> .bf; 

    rel(area.boundaryarea)[admin_level=8];
    map_to_area -> .bllf;
    
    (.bllf - .bf ) ; 
    rel(pivot); (._;>;); out;


http://overpass-turbo.eu/s/56K 

![map_to_area](https://cloud.githubusercontent.com/assets/5842757/4344274/33344c46-4085-11e4-8ff8-ae7043229b8d.png)



Code may need some further polishing. Attic part was not tested due to missing test data on local instance.

Feedback welcome!