Add Spectral colour map.

Probably, it was assumed that this colour map is the same as 'spectral',
but that is not the case. *S*pectral is diverging, while *s*pectral is
miscellaneous. Also, spectral has been renamed to nipy_spectral.
Rename 'spectral' to 'nipy_spectral'.

This is the name used in the current matplotlib, and probably better
reflects its origin.