Implement printing POIs

[mowgli]

I had to change the way, text is written to the map to allow utf-8 encoded strings.

closes #98

[mowgli]

The build failure is due to a too old libgd-dev.

The current version (2.3.3) has the following definition:

BGD_DECLARE(char *) gdImageStringFT (gdImagePtr im, int *brect, int fg, const char *fontlist,
                                     double ptsize, double angle, int x, int y,
                                     const char *string);

So the last argument keeps his const.

I see nothing I can do against as I cannot update the github build environment.