Add comments, tweak function names
Remove blank line
Add cache-busting for require.js for development
Tweak allocation of colors to lines
Add todo
Tidy usage of line IDs
Better allocate colors of lines.

Rather than using a d3.colorScale, we now have a pool of colors and assign
a color to each line's data when the data is prepared.

This means we can get the next color, release now unused colors (when a line
is deleted), and have the color data available for use when drawing the
line's key in mappiness.ui.