new theme: monochrome

[Xavier Noria]

Hi there!

I've written this gray on black theme called monochrome (screenshot in
the home):

    https://github.com/fxn/monochrome-theme.el

This is my first theme ever, and started with the source code of the
builtin dichromacy.

If you like it please feel free to add it, that'd be an honor :).

Xavier

PS: I will probably tweak it a bit before Emacs 24 is released.