Appveyor for Windows CI (#574)

* added basic AppVeyor config

* [AppVeyor] /verbosity:minimal

appveyor.yml

+version: "{build}"
+
+configuration: Release
+
+build_script:
+  # build test
+  - mkdir %APPVEYOR_BUILD_FOLDER%\build_test
+  - cd %APPVEYOR_BUILD_FOLDER%\build_test
+  - cmake -G "Visual Studio 14 2015 Win64" ../dlib/test
+  - cmake --build . --config %CONFIGURATION% --target dtest -- /m /verbosity:minimal
+  # build examples
+  - mkdir %APPVEYOR_BUILD_FOLDER%\build_examples
+  - cd %APPVEYOR_BUILD_FOLDER%\build_examples
+  - cmake -G "Visual Studio 14 2015 Win64" ../examples
+  - cmake --build . --config %CONFIGURATION% -- /m /verbosity:minimal
+
+test_script:
+  # run test
+  - cd %APPVEYOR_BUILD_FOLDER%\build_test\%CONFIGURATION%
+  - dtest --runall
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