Class Image now correctly finalizes its unmanaged data.

Instances of Image will now call gdImageDestroy() on the gdImagePtr they
hold when garbage-collected.  Previously, they did not.
Added DotNet-style XML comments and make rules to build them.

The build process now creates HTML documentation for the library.

Docs are minimal.  The user is expected to refer to the equivalent GD
manual entries.
Removed default value from ImageData 'type' constructor argument.

ImageData's public constructor takes an image type as its second
argument.  This used to default to PNG if not given.  It is now
required.