Add yarn package

packages/yarn.rb

+require 'package'
+
+class Yarn < Package
+  description 'Yarn is a new package manager for JavaScript and an alternative to npm.'
+  homepage 'https://yarnpkg.com/en/'
+  version '0.27.0-20170629.1443'
+  source_url 'https://nightly.yarnpkg.com/yarn-v0.27.0-20170629.1443.tar.gz'
+  source_sha256 'e1086d267151cd05896b15023e8c4e253a64e675a80bb38dc294b5b1c86c6656'
+
+  def self.install
+    system "mkdir -p #{CREW_DEST_DIR}/usr/local"
+    system "cp -r lib/ #{CREW_DEST_DIR}/usr/local"
+    system "cp -r bin/ #{CREW_DEST_DIR}/usr/local"
+  end
+end