Configuration

Structure cost, build time, maximum amount of citizens and revenue is now configurable through a json file. See:

src/main/config/structureproperties.json  (as per maven project structure guidelines: https://maven.apache.org/guides/introduction/introduction-to-the-standard-directory-layout.html)

Note that only the revenue is actually used at this point.

*Important*

The way it is implemented now, the file is read and parsed  at every poll request _for every structure_ (!!). This is obviously a performance issue that has to be resolved later. I suggest this information has to be read during server startup and stored in the application cache or something. For now it works. If anyone wants to do this, be my guest

Fixes #126