Add support for colors with transparency in palettes and legend.

[joanpau]

Currently, palettes are composed of opaque colors only. In addition the legend is always displayed with black background and white text. It would be desirable to be able to request a legend with a custom background color or tranparent, and also to specify colors with alpha values in the palettes. This proposal introduces both improvements, with complete backwards compatibility:

• Improve handling of color parameters in the requests.
• Modify the color model generation to handle transparent colors. The alpha value of each color is scaled by the opacity value.
• Modify the color palette reader to support alpha channel values in each color definition line (ARGB order). If a line has only three values (RGB order) the color is completely opaque, providing full compatibility with existing palettes without alpha values in their colors.
• Add transparent and bgcolor parameters to the legend request to control the appearance of the background of the legend. This are the same parameters used in a GetMap request. They default to false and black, so the handling of the legend request is backwards compatible if the parameters are omitted.

[lesserwhirls]

Again, sorry for the delay, and thanks for the pull request!