Add background variants for term-color-*

These have defaults which don't look right in ansi-term etc.

They're also safe to overwrite the background color of because each variant applies to the ANSI equivalent of background/foreground, e.g text won't suddenly become entirely orange.

Without this change you get yellow background like the one seen on `master`:

![Screen Shot 2013-03-31 at 19 40 28](https://f.cloud.github.com/assets/1516/322216/7e47da86-9a32-11e2-97f9-fb47eda5d13f.png)

Prior to this change running `M-x describe-face term-color-yellow` shows the following:

    Face: term-color-yellow (sample) (customize this face)
    ...
    Foreground: #a57705
    Background: yellow3