Map.rotate() improvements

Changes:
* `Map.rotate()` now preserves all image data instead of cropping!
* For accuracy, the default `Map.rotate()` transformation is now bi-quartic instead of bi-cubic (see discussion starting at https://github.com/sunpy/sunpy/pull/1074#issuecomment-49390071).  Folks who want to prioritize speed over accuracy can explicitly specify a lower order of transformation.
* Fixed a bug when specifying a fill value for skimage transforms
* Settled on `rotation_center` for the name of the argument in `Map.rotate()` (closes #1112)