Display multichannel image

In microscopy, often images have multiple color channels, each having the fluorescence intensity of a certain dye emission. The function `to_rgb` converts these multichannel images to RGB colors. Either the metadata `color` field is used for the colors, or default colors cyan / green / magenta / red.

This is implemented in the `_repr_html_` function, to allow direct views of multichannel images.

Earlier on, I used http://www.noah.org/wiki/Wavelength_to_RGB_in_Python to convert emission wavelengths to RGB values. Because licensing is unclear and I get no response on my emails, I removed this code and changed the wavelength designation to matplotlib colors such as `'r'` or `[0.2,0.0,1.0]`.
I would just leave this as float and only cast back to uint8 at the very end.  It turns out some of the alpha strangeness in mpl is actually due to always going through uint8 buffers.  It is better to keep all of the intermediaries as float to reduce rounding errors and avoid sloshing data back and forth in memory more than strictly needed.