Add support for selecting a color palette

[loopj]

Instead of black text on the same gray background, generate consistent background colors for each set of initials.

• Allow users to request a pre-defined palette name (pastel, neon, etc)
• Generate a couple of palettes to choose from, using i want hue or color brewer
• Use consistent hashing to map initials to one of the colors in the selected palette
• Allow selecting the palette by filename (initials-size-palette.png) or querystring (initials.png?palette=blah)