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
Fix recenter for cube slicing
Whitespace
Merge pull request #122 from astrofrog/fix-pixel-scale

FITSFigure.recenter now uses radius in pix, not deg anymore