feature request: pointInPolygon() and pointOnVertex() in Polygon.class.php

I added the functions pointInPolygon() and pointOnVertex() to Polygon.class.php

I'm not sure if this is an appropriate place to put them, or if it's something that falls under this project's purview, but I found it useful for a project i'm working on.

Adapted from http://www.assemblysys.com/dataServices/php_pointinpolygon.php