add appveyor for CI on windows

.appveyor.yml

@@ -0,0 +1,19 @@
+environment:
+  matrix:
+  - TARGET: x86_64-pc-windows-msvc
+    platform: x64
+  - TARGET: i686-pc-windows-msvc
+    platform: x86
+
+install:
+  - appveyor-retry appveyor DownloadFile https://win.rustup.rs/ -FileName rustup-init.exe
+  - rustup-init.exe -y --default-host %TARGET%
+  - set PATH=%PATH%;C:\Users\appveyor\.cargo\bin
+  - rustc -V
+  - cargo -V
+
+# Disable AppVeyor's build phase, let 'cargo test' take care of the build
+build: false
+
+test_script:
+- cargo test --all --target %TARGET%