add xterm-color colors

xterm-color.el might not work in Emacs 24, it didn't work for me when
copy/pasting instructions anyway.
(hopefully) a better ansi-term mapping

Eterm is an 8 color terminal.

I visually compared the result of these changes with some terminal
outputs, I especially tried to get output of color-chart to be as close
as possible to my rxvt-256 terminal.