New Polygon Clipping and Offsetting

Provide a front end to Angus Johnson's polygon clipping and offsetting algorithms.
Two functions are provided:
* polyclip(x1,y1,x2,y2,method) finds the difference/intersection/xor/union (depending on "method") between two polygons
* polyout(x1,y1,delta,join,joininfo) outsets a polygon by distance delta, using the given corner join method (and optional joininfo specifying more details about the corner) [corner is 'round' or 'square' or 'miter']. Insetting is accomplished with a negative delta.
See the README.txt file for where to download Angus Johnson's clipper C++ code and for compiling instructions.