Adding additional build environments for AppVeyor

Signed-off-by: Remigiusz Kołłątaj <remigiusz.kollataj@gmail.com>

appveyor.yml

+version: 1.0.{build}
+image: Visual Studio 2015
+environment:
+  matrix:
+  - GENERATOR: '"MinGW Makefiles"'
+    BUILD_TYPE: Debug
+  - GENERATOR: '"MinGW Makefiles"'
+    BUILD_TYPE: Release
+  - GENERATOR: '"Visual Studio 14 2015"'
+    BUILD_TYPE: Debug
+  - GENERATOR: '"Visual Studio 14 2015"'
+    BUILD_TYPE: Release
+  - GENERATOR: '"Visual Studio 14 2015 Win64"'
+    BUILD_TYPE: Debug
+  - GENERATOR: '"Visual Studio 14 2015 Win64"'
+    BUILD_TYPE: Release
+build_script:
+- cmd: >-
+    set
+
+    mkdir build
+
+    cd build
+
+    set PATH=%PATH:C:\Program Files\Git\usr\bin;=%
+
+    set PATH=C:\mingw-w64\i686-5.3.0-posix-dwarf-rt_v4-rev0\mingw32\bin;%PATH%
+
+    cmake .. -G %GENERATOR% -DCMAKE_BUILD_TYPE=%BUILD_TYPE%
+
+    cmake --build . --config %BUILD_TYPE%
+test: off