clean up some code style
magickio.c: free rgb memory
magickio.c: add rgb and rgba function

The gray function now calls into the rgba function rather than doing the
grayscale conversion and image loading in the same place...reuse the rgba
function instead.
fileio.c: use less memory
compute_asift_keypoints.cpp: mark some vars and funcs static
first step to encapsulate keypoints structure