wip: Array.to_rgb

[kylebarron]

This is a bit messy because:

• We have a bit of a conflicting theme of whether the output should be display ready. E.g. does the output dtype always have to be uint8?
• Does the input data type always have to be uint8?
• For grayscale, we assume that the max value is 255
• For paletted images we have some assumptions over the data type of the data

Closes #25