Simplify ncursesw build process

This patch simplifies the ncurses build process by
1. unconditionally including curses.h on all distributions; and
2. removing all probing logic from ncursesw/Setup.lhs.

curses.h exposes wide character variants and types when
`NCURSES_WIDECHAR=1` (implied by `_XOPEN_SOURCE_EXTENDED`).
Assuming the above holds on all recent ncurses installs,
we can simplify mycurses.h by unconditionally including
only curses.h.

The standard location for the ncurses header is
/usr/include/curses.h, but this may vary across distributions.
Figuring out where ncurses is actually installed is a task
best left to package vendors.
Remove all probe logic from Setup.lhs and let the user
set --extra-lib-dirs and --extra-include-dirs as needed
to accommodate non-standard ncurses installs.
README: need patched version of setlocale for GHC 7.8
System.Cmd is deprecated
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
default.nix: build with GHC 7.8.3
default.nix: use latest revision of libmpd-haskell