Add a new method, attach_gd, which attaches a tclgd object.

Change the attach method name to attach_spi.

Extend the pilights clientData structure to include a pointer to a gd image (gdImagePtr).

Change the calling parameters to copy_from_image to take a start row, number of rows, image y, x, start pixel and number of pixels.

Fix bug and cleanup write method -- not sure how it ever worked.

Add comments.

Update doc.

New playpen program, play.tcl.