Shift matrix

Fast and convenient function for shifting a matrix in 2D.

The code uses array indexing to implement shifting of the matrix. It is about 2.5 times faster than using interp2 with 'nearest neighbor' option for shifting of the matrices. Inspiration for optimized array indexing comes from a snippet on circular shift in: "MATLAB array manipulations tips and tricks" guide by Peter J. Acklam (http://home.online.no/~pjacklam).