ENH: Convert multichannel to RGB

- adds function `to_rgb` that converts a multichannel N-dimensional
image to RGB image
- implements this in the `_repr_html_` function
Minor adaptations, docstrings
Removed wavelength to rgb

In stead of wavelength -> rgb, use matplotlib color -> rgb conversion.
Wavelength conversion code had unclear licensing.
Metadata tag 'color' is used, changed defaults

- default colors are now cyan/green/magenta/red
- color metadata is checked and used
Made _monochannel_to_rgb global
Changes after line notes

- added clipping step, when colors overlap too much, renormalizing would
reduce the luminosity of the channels. When clipping needs to be
avoided, the user can specify his own RGB values (see docstring)
- when normalize == True, also monochannel images are normalized
- moved import ColorConverter to module level
- defaults colors are now directly rgb to circumvent ColorConverter
- more efficient _monochannel_to_rgb
Removed unnecessary -> int conversion

- calculations are in floats, and only when normalize = True the
datatype is converted to np.uint8 at the end
- mistake in _repr_html_ for display of 2D multichannel
Cleanup

- made PEP8 compatible
- `colors == None` becomes `colors is None`
- replaced undefined variable `i` with `0`
Merge pull request #120 from caspervdw/display-multichannel

Display multichannel image