Added support for NChannelImage and ExtendToNChannel


Sorry for all the previous open/close pull requests. This is the only merge to look at.

I can't find ``RGBImage``. It isn't currently in imagen or your commit to featuremapper. The only thing that I cna find that seems related is [jitter_hue](https://github.com/ioam/featuremapper/blob/master/featuremapper/colorspaces.py#L503-507).