Fixed rotation being around the center rather than the origin.

Probably because I wasn't thinking of it, I never made rotation
be about the origin; it just rotated about the center every time.
This is fixed now.
Somewhat fixed broken rotation.

The last commit completely destroyed rotation.  This commit tries
to help, though it's still completely wrong.  See
examples/rotation.py; the needles are supposed to rotate around the
heads.

The problem that still exists is somewhere between lines 1652 and
1697 of StellarClass.py (the new code which is supposed to make
images rotate about the origin).