Structured search requests

Adding structured search options to Nominatim::Search.

As described in http://wiki.openstreetmap.org/wiki/Nominatim#Parameters, you can send structured search requests using street, city, county, state and country parameters.

This commit adds these methods to the Nominatim::Search. Since you cannot mixed structured requests
with the q parameter, if an structured search is performed, the q criterion is deleted before
performing the request.