astrometry tweak

I spent today on issues related to getting our tweak code up and working on the WWW. The tweak code takes an image with reasonable astrometric WCS (good to a few arcmin), and replaces it with more accurate WCS (good to better than an arcsec), computed at user-set polynomial order, and puts that more accurate WCS into the image header in a standards-compliant way. This will not be a unique capability of astrometry.net, but it is essential to its future, unique capabilities.