added a "bugfix" / hack to copy pc -> cd
added test for wcs hack, and added more layers of checking for pc vs cd
(i.e., in scale).  Use if+hasattr instead of try b/c nested try/excepts
are ugly.
commented out a failing test.  Tests still do not pass.
removed hack, which really seriously broke wcs, and instead use a more
sensible replacement of the pixel_scale function
hasattr(..,'cd') -> .has_cd()
Merge pull request #116 from keflavich/pc-cd-bugfix

added a "bugfix" / hack to copy pc -> cd