Create Appveyor config for automated CI testing

appveyor.yml

@@ -0,0 +1,29 @@
+version: '{build}'
+install:
+- cmd: >-
+    mkdir thirdparty
+
+    cd thirdparty
+
+    set CMAKE_URL="https://cmake.org/files/v3.8/cmake-3.8.0-win64-x64.zip"
+
+    appveyor DownloadFile %CMAKE_URL% -FileName cmake.zip
+
+    7z x cmake.zip -o%cd% > nul
+
+    move cmake-* cmake
+
+    set PATH=%cd%\cmake\bin;%PATH%
+
+    cmake --version
+
+    cd ..
+build_script:
+- cmd: >-
+    mkdir build
+
+    cd build
+
+    cmake .. -DCMAKE_INSTALL_PREFIX="C:\Program Files (x86)\DiscordRPC"
+
+    cmake --build . --config Release --target install