Changed the norm() method of GenericMap to an attribute.

This allows the user to overwrite it and customise it.
I have also changed the keyword parseing in MapCube.plot()
so that all kwargs are passed to imshow and kwargs to
mpl.animation.FuncAnimate can be passed in a dict in the ani_kwagrs
kwarg.
Merge branch '0.3_norm' into norm_fix
Merge remote-tracking branch 'upstream/master' into norm_fix

Conflicts:
	sunpy/map/mapbase.py
Merge pull request #596 from Cadair/norm_fix

Changed the norm() method of GenericMap to an attribute.