overloaded `_make_twin_axes` on `LocateableAxesBase` so that twinx and
twiny will work.
Remove redundant colormaps from examples

`gist_yarg`, `gist_gray`, and `binary` are identical to `gray`, so they are "on a deprecation path", and `spectral` was renamed to `nipy_spectral` to avoid conflicts with `Spectral`

https://github.com/matplotlib/matplotlib/pull/889
DOC: Move spectral colormaps together

`Spectral` was missing, and `hsv` and `nipy_spectral` shouldn't be in the qualitative group, which are the 8 colormaps dumped here from ColorBrewer but that shouldn't really be here
Merge pull request #2659 from endolith/patch-3

DOC: Remove redundant colormaps from examples
Merge pull request #2515 from tacaswell/locatable_twin

overloaded `_make_twin_axes` on `LocateableAxesBase`