Add rust-nightly

Just started being released:
https://mail.mozilla.org/pipermail/rust-dev/2014-March/009295.html

Casks/rust-nightly.rb

@@ -0,0 +1,12 @@
+class RustNightly < Cask
+  arch = `uname -m`.chomp
+  arch = 'i686' if arch.empty?
+  pkgname = "rust-nightly-#{arch}-apple-darwin.pkg"
+
+  url "http://static.rust-lang.org/dist/#{pkgname}"
+  homepage 'http://www.rust-lang.org/'
+  version 'latest'
+  install pkgname
+  no_checksum
+  uninstall :pkgutil => 'org.rust-lang.rust'
+end