add appveyor

appveyor.yml

@@ -0,0 +1,12 @@
+version: '{build}'
+os: Visual Studio 2015
+init: []
+install: []
+build_script:
+- set PATH=C:\Program Files (x86)\MSBuild\14.0\Bin;%PATH%
+- mkdir build
+- cd build
+- cmake -G "Visual Studio 14 2015 Win64" -D SHARED=1 -D BUILD_TESTS=1 ../cmake
+- cmake --build . --config Debug
+test_script:
+- bin\xlnt.test.exe