Places / refactoring service to allow the search of more than one ID.

This should dramatically reduce the number of queries done to the search server.
Attempt to make code a bit easier to read...
Old API as well as new one, added test to cover both cases.
Placed get_places_by_identifiers into own test

Silly test but nice to have 1 assertion in each test
Fix recursion error

Must have been a typo from @martinfilliau
Merge pull request #15 from ox-it/refactor_get_pois

Places / refactoring get pois