Make importshp plugin read polygons

Tested on the following data sets from naturalearthdata.com:

http://www.naturalearthdata.com/http//www.naturalearthdata.com/download/110m/cultural/ne_110m_admin_0_countries_lakes.zip
http://www.naturalearthdata.com/http//www.naturalearthdata.com/download/110m/physical/ne_110m_land.zip
http://www.naturalearthdata.com/http//www.naturalearthdata.com/download/110m/physical/ne_110m_ocean.zip

Signed-off-by: Tamas TEVESZ <ice@extreme.hu>