Add continuous integration on Windows through AppVeyor

Only test on Node 4 (latest LTS), contrary to Travis which runs Node 4
and 6.

Author: tbroyer

appveyor.yml

@@ -0,0 +1,15 @@
+# See https://www.appveyor.com/docs/lang/nodejs-iojs
+environment:
+  nodejs_version: "4"
+
+install:
+  - ps: Install-Product node $env:nodejs_version
+  - node --version
+  - npm --version
+  - npm install
+
+test_script:
+  - npm test
+
+build: off
+