add appveyor.yml: test both vc2015 and vc2017

Author: minad

appveyor.yml

@@ -0,0 +1,11 @@
+version: 1.0.{build}
+image:
+- Visual Studio 2017
+- Visual Studio 2015
+build_script:
+- cmd: >-
+    if "Visual Studio 2017"=="%APPVEYOR_BUILD_WORKER_IMAGE%" call "C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Auxiliary\Build\vcvars64.bat"
+    if "Visual Studio 2015"=="%APPVEYOR_BUILD_WORKER_IMAGE%" call "C:\Program Files\Microsoft SDKs\Windows\v7.1\Bin\SetEnv.cmd" /x64
+    if "Visual Studio 2015"=="%APPVEYOR_BUILD_WORKER_IMAGE%" call "C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat" x86_amd64
+    nmake -f makefile.msvc all
+    test.exe