WIP: Added initial image reprojection functionality

This was discussed during SciPy as a possible feature to add to astropy. This routine would act as a high-level wrapper for interpolating reprojection, but also a wrapper to the drizzle algorithm for flux-conserving reprojection, and even a Montage-like algorithm. This is of course work in progress, but feedback about this idea is already welcome (will post to the list once it's a little more functional).