(intended for extended emission, not stellar images!)
Documentation: http://image-registration.rtfd.org
Borrows heavily from http://www.mathworks.com/matlabcentral/fileexchange/18401-efficient-subpixel-image-registration-by-cross-correlation/content/html/efficient_subpixel_registration.html
and replicates much of the functionality of http://www.astro.ucla.edu/~mperrin/IDL/sources/subreg.pro
and implements http://solarmuri.ssl.berkeley.edu/~welsch/public/software/cross_cor_taylor.pro
