Another branch to get rid of random commits

Some PEP8 fixes pulled from another PR to ease merging

Only update the CROTA2 keyword if it already exists

Updated map.rotate() tests

map.rotate() docstring tweaks

Re-implemented recenter functionality

Update the rotation angle in the metadata, and use by default

Proper rotation of rectangular maps
Merge branch 'master' of https://github.com/mateoi/sunpy into cubes