.dzen: retire dzen

It's too much of a hack.  Let's get xmobar or something later.
.Xdefaults: switch font to Droid Sans Mono
.xmonad: configure xmonad for real

Major changes:
1. Use combineTwo to combine two simpleTabbed layouts with a TwoPane.
   The only other layout we need is Full.
2. Use XMonad.Layout.WindowNavigation for intuitive navigation.
3. Use yeganesh, a nice wrapper around dmenu.
4. Spawn xmobar on startup.  Use DynamicLogger to pretty-print to
   xmobar.
5. Rename workspaces so they show up prettily on xmobar.
.xmobarrc: introduce xmobar configuration

For some reason, `Run MPD` doesn't work.  Investigate this in the
future.