Add support for certain stereoplotters

Requested by Ulf Almroth at the QGis user list
http://comments.gmane.org/gmane.comp.gis.qgis.user/8077

A lot of measuring devices generating ascii files with coordinates, e.g. various type of surveying equipment and coordinate measuring devices for industrial use produces file that basically look like this:

featuretyp, X, Y, Z, pencode

Think of polygons representing landuse or cadstral divisons or lines representing streams or roads. In this simple file structure a line looks like this:

STOLP 1868536.305 7323297.568 5.206 21
STOLP 1868514.063 7323234.352 2.558 22
STOLP 1868445.482 7323050.904 3.220 22
STOLP 1868514.063 7323234.352 2.558 23

The polygon differs rom the line in that the points with pencode 21 & 23 have the same coordinates indicating the closure. Point have the pencode 11 indicating no connection:

AHP,1868280.000,7323450.000,0.271,11
AHP,1868300.000,7323450.000,1.324,11
AHP,1868320.000,7323450.000,0.542,11