Add support for AppVeyor.

It will only build with Visual Studio 2017:
- Build in Release mode
- Build in Debug mode
- Test the installation

README.md

@@ -1,4 +1,5 @@
 [![Build Status](https://travis-ci.org/Exiv2/exiv2.svg?branch=master)](https://travis-ci.org/Exiv2/exiv2)
+[![Build status](https://ci.appveyor.com/api/projects/status/yqtbcfb60588ogsa?svg=true)](https://ci.appveyor.com/project/piponazo/exiv2)
 
 <pre><code>
     @@@Marco@@@@@b                   ;mm                       /##Gilles###\

appveyor.yml

@@ -0,0 +1,10 @@
+image:
+- Visual Studio 2017
+
+build_script:
+  - md build
+  - cd build
+  - cmake -DEXIV2_ENABLE_XMP=OFF -DEXIV2_ENABLE_NLS=OFF -DEXIV2_ENABLE_PNG=OFF -DCMAKE_INSTALL_PREFIX=install ..
+  - cmake --build . --config Release
+  - cmake --build . --config Debug
+  - cmake --build . --target install