Extended polygon statement - nodes inside a closed way

This pull request aims at closing the gap between the current lat/lon pair based polygon statement and the area statement, which isn't available for all osm objects. Furthermore, I've added a statement dump implementation for `polygon`. A few use cases are also provided in #77. 

This PR may need a little cleanup. Especially method convert_inputset has a few calls to other methods I've taken over from make_area.cc. Not sure if this all makes sense.

Fixes #77