Merge pull request #147 from astrofrog/update-from-template-3

Update from template
Use ``get_cdelt`` to find pixel scale instead of trying to do something different depending on the internal representation. This fixes a bug which occurred if the PC matrix was normalized.
Forgot to take the absolute value of cdelt
Clarify units for recenter, and differentiate between cases where cdelt values will be different for both axes, and cases where they won't.
Added changes missing in previous commit