demos/poi_backend updated to conform to the latest POI-DB API specifications

Some code refactoring done all around.

backendServer_http.py:
* Updated to the latest POI-DB API specification
* Added multiprocessing support enabling the backend to efficiently
  utilize multiple CPUs
* Added gzip support to reduce the size of the returned data

poi_data_manager.py:
* Improved security against SQL injections using psycopg2's execute()
  method for passing variables, see http://initd.org/psycopg/docs/usage.html#the-problem-with-the-query-parameters