#522: Initial commit for 8-bit RAW support.  This commit provides support for file import (but not from camera), 
JPEG mimicking (to prevent full-sized RAW decodes and processing whenever possible), and a file format 
abstraction layer in anticipation of PNG and TIFF support.

Some RAW files do not display correctly; some have odd dimensions, other have color problems.  I'm investigating 
these currently.  Also, to improve import time, instead of displaying a full-window pixbuf of each photo as it's 
loaded, the largest thumbnail is displayed instead.  This behavior can be reinstated, if deemed important.