libjpeg API: Partial scanline decompression

libjpeg API: Partial scanline decompression

This, in combination with the existing jpeg_skip_scanlines() function,
provides the ability to crop the image both horizontally and vertically
while decompressing (certain restrictions apply-- see libjpeg.txt.)

This also cleans up the documentation of the line skipping feature and
removes the "strip decompression" feature from djpeg, since the new
cropping feature is a superset of it.

Refer to #34 for discussion.

Closes #34