add appveyor

.appveyor.yml

@@ -0,0 +1,15 @@
+os: 
+  - Visual Studio 2015
+  - Visual Studio 2013
+
+platform:
+  - Win32
+  - Win64
+
+before_build:
+  - ps: Invoke-Command -ScriptBlock ([ScriptBlock]::Create((Invoke-Webrequest "https://raw.githubusercontent.com/tboox/xmake/dev/scripts/get.ps1" -UseBasicParsing).Content)) -ArgumentList "repo"
+  - cmd: xmake --version
+
+build_script:
+  - cmd: xmake l scripts/test.lua
+