v0.6.0

Stop treating the alpha channel specifically in the Euclidean distance formula. Instead, assume it's always the fourth channel and assign it a default weight of 0. This means that it's now possible to factor in the alpha into the distance, if needed.

There's also a change of handing NaN differences on a channel. In this case, the distance on that particular channel is considered to be zero. (Which kind of makes sense?)

Distances on the hue (h channel) are now computed with the shortest hue distance, taking into account the cyclical nature of this channel. (#68).