First commit of desert256v2 theme

The desert256v2 is a modified version of the original (and excellent)
"desert" vim theme.  I've been using this theme for a few years and over
this time I made what I believe to be a few improvements to the original
theme:

- Diff colors are now easier to see (follow a bit the colors used by
  tkdiff)

- Since vim resets the "background" variable based on the background
  color, the original theme is meant as a dark theme, but background
  ends up being reset to "light" by vim. I made the background darker to
  avoid this problem.

- I also changed the colors for search highlight, visual selection, and
  other minor aspects of the interface.