Move the code from appveyor config to a Python script

No more PowerShell rubbish, yay!

appveyor.yml

@@ -10,37 +10,8 @@ environment:
   - BUILD: mingw
     CONFIG: Release
 
-install:
-  - ps: |
-      if ($env:BUILD -eq "mingw") {
-        # Install MinGW.
-        $url  = "http://sourceforge.net/projects/mingw-w64/files/"
-        $url += "Toolchains%20targetting%20Win64/Personal%20BUILDs/"
-        $url += "mingw-builds/4.9.2/threads-win32/seh/"
-        $url += "x86_64-4.9.2-release-win32-seh-rt_v3-rev1.7z/download"
-        Invoke-WebRequest -UserAgent wget -Uri $url -OutFile mingw.7z
-        &7z x -oC:\ mingw.7z > $null
-      }
-  - set PATH=C:\Program Files (x86)\MSBUILD\12.0\bin\;%PATH%;C:\mingw64\bin
-
-before_build:
-  - python support/build.py
-
 build_script:
-  - ps: |
-      if ($env:BUILD -eq "mingw") {
-        mingw32-make -j4
-      } else {
-        msbuild /m:4 /p:Config=$env:CONFIG FORMAT.sln
-      }
-
-test_script:
-  - ps: |
-      if ($env:BUILD -eq "mingw") {
-        mingw32-make test
-      } else {
-        msbuild RUN_TESTS.vcxproj
-      }
+  - python support/build.py
 
 on_failure:
   - appveyor PushArtifact Testing/Temporary/LastTest.log