New distance-calculations proposal

[nicklas]

A hopefully more structured ticket than #137

The patch attached includes:
new functions:

1. st_shortestline
   
2. st_longestline
   
3. st_dfullywithin
   

enhancements affecting st_distance, st_dwithin and the functions above:

1. a better handling of "holes" in polygons.
   
2. a sudden stop in iteration when the distance in st_dwithin is satisfied. Before it was checked for every subgeom.
   
3. a new way of calculating distances between lines and polygons that don't intersect.
   

Probably the last one is the most important. On big geometries it makes a big difference.

Beside the patch I have attached a document where I try to explain the idea behind the distance-calculation.

/Nicklas

[nicklas]

explanation of the distance calculation