Estonia, Latvia, Lithuania

According to [UPU for Estonia](http://www.upu.int/fileadmin/documentsFiles/activities/addressingUnit/estEn.pdf):

> For large towns, the district is defined by the town and the post office. In other cases,
> the district is defined by the region and the rural municipality.

This means that people in small towns write the region instead of the locality. Because of this, I've been getting bug reports and requests from Estonian users about adding the administrative_area (region) field for Estonia.

Based on that, it makes sense to me to add a non-required administrative_area field for Estonia, and define the 15 Estonian regions.