License ncursesw.

MIT/X11 as is ‘ncurses’ itself. Closes #71.
README: document prereqs for building ncursesw
Add default.nix

With the nix package manager you can do

   $ nix-shell

to get a working environment containing all
you need to hack and run vimus.
nix: use latest revision of libmpd-haskell
default.nix: specify isLibrary/isExecutable
Refactor default.nix
default.nix: fetch git setlocale for 7.8 support