chore: Add windows CI (appveyor) (puppeteer#1046)

This patch adds appveyor to make sure puppeteer works reliably on
windows.

Fixes puppeteer#955

.appveyor.yml

@@ -0,0 +1,19 @@
+environment:
+  matrix:
+    - nodejs_version: "6"
+    - nodejs_version: "7"
+
+build: off
+
+install:
+  - ps: Install-Product node $env:nodejs_version
+  - yarn install
+  - if "%nodejs_version%" == "7" (
+      yarn run lint &
+      yarn run coverage  &
+      yarn run test-doclint
+    ) else (
+      yarn run test-node6-transformer  &
+      yarn run build  &
+      yarn run unit-node6  
+    )

README.md

@@ -1,4 +1,4 @@
-# Puppeteer [![Build Status](https://travis-ci.org/GoogleChrome/puppeteer.svg?branch=master)](https://travis-ci.org/GoogleChrome/puppeteer) [![NPM puppeteer package](https://img.shields.io/npm/v/puppeteer.svg)](https://npmjs.org/package/puppeteer)
+# Puppeteer [![Build Status](https://travis-ci.org/GoogleChrome/puppeteer.svg?branch=master)](https://travis-ci.org/GoogleChrome/puppeteer) [![App Veyor build status](https://ci.appveyor.com/api/projects/status/github/aslushnikov/puppeteer)](https://ci.appveyor.com/project/aslushnikov/puppeteer) [![NPM puppeteer package](https://img.shields.io/npm/v/puppeteer.svg)](https://npmjs.org/package/puppeteer)
 
 <img src="https://user-images.githubusercontent.com/10379601/29446482-04f7036a-841f-11e7-9872-91d1fc2ea683.png" height="200" align="right">