Add appveyor config

The configuration is based on the sample from [here][1]. Since Appveyor
does not run jobs in parallel it is currently only running the tests
against stable rust to avoid slowing down the builds too much.

[1]: https://github.com/starkat99/appveyor-rust

appveyor.yml

@@ -0,0 +1,23 @@
+os: Visual Studio 2015
+
+environment:
+  REDISRS_SERVER_TYPE: tcp
+  RUST_BACKTRACE: 1
+  matrix:
+    - channel: stable
+      target: x86_64-pc-windows-msvc
+    - channel: stable
+      target: x86_64-pc-windows-gnu
+install:
+  - appveyor DownloadFile https://win.rustup.rs/ -FileName rustup-init.exe
+  - rustup-init -yv --default-toolchain %channel% --default-host %target%
+  - set PATH=%PATH%;%USERPROFILE%\.cargo\bin
+  - rustc -vV
+  - cargo -vV
+  - cmd: nuget install redis-64 -excludeversion
+  - set PATH=%PATH%;%APPVEYOR_BUILD_FOLDER%\redis-64\tools\
+
+build: false
+
+test_script:
+  - cargo test --verbose %cargoflags%