CD matrix now updated, if present, when Map pixel size is changed.

See https://github.com/sunpy/sunpy/pull/1145#issuecomment-56837577.  This PR fixes the immediate bug with `resample()`/`superpixel()` by making sure that the CDELTs and the CD matrix stay in sync, and defers until later a decision on how we actually want to handle this redundancy and the possible non-presence of CDELTs.