Add debug appveyor configuration

.appveyor.yml

@@ -2,6 +2,9 @@ version: 1.0.{build}
 os: Visual Studio 2015
 clone_folder: C:\projects\pybind11
 test: off
+configuration:
+    - Release
+    - Debug
 branches:
   only:
     - master
@@ -18,6 +21,6 @@ build_script:
   - cd c:\projects\pybind11
   - cmake -G "%CMAKE_PLATFORM%" -DPYTHON_INCLUDE_DIR:PATH=%PYTHON_DIR%/include -DPYTHON_LIBRARY:FILEPATH=%PYTHON_DIR%/libs/python34.lib -DPYTHON_EXECUTABLE:FILEPATH=%PYTHON_DIR%/python.exe
   - set MSBuildLogger="C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll"
-  - set MSBuildOptions=/v:m /p:Configuration=Release /logger:%MSBuildLogger%
+  - set MSBuildOptions=/v:m /p:Configuration=%Configuration% /logger:%MSBuildLogger%
   - msbuild %MSBuildOptions% pybind11.sln
-  - ctest -C "Release"
+  - ctest -C %Configuration%