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]`.
We *do* want to import this inside the function, not at the module level, because mpl is an optional dependency of PIMS. But, sure, for the sake of clarity / easy maintenance, import it at the top of the function.