Refine API parameters: replace parent with combination of region & level

[simonwoerpel]

• drop the parent parameter
• make the level parameter array-like to get trees

examples:

region=01,03&level=3 - get all districts ("Kreise") from "Bundesländer" 01 and 03
region=01,03&level=1: - get complete tree starting from "Bundesländer" level
region=01,03&level=1:3 - get tree excluding lau level
region=01001&level=:3 - get upwards tree starting from district 01001
region=02&level=2,4 - get all "stat. Regionen" and "Gemeinden" for "Bundesland" 02