AppVeyor: add scripts for Coverity Scan build submission

I thought we could use AppVeyor due to it's higher timeout of 1 hour, but it's stuck on lib compilation.
I'll still keep scripts in place so we can use them in future in case something changed

CI/msvc/coverity_build_script.bat

@@ -0,0 +1,5 @@
+cd %APPVEYOR_BUILD_FOLDER%
+cd build_%VCMI_BUILD_PLATFORM%
+
+echo Building with coverity...
+cov-build.exe --dir cov-int cmake --build . --config %VCMI_BUILD_CONFIGURATION% -- /maxcpucount:2

CI/msvc/coverity_upload_script.ps

@@ -0,0 +1,17 @@
+7z a "$Env:APPVEYOR_BUILD_FOLDER\$Env:APPVEYOR_PROJECT_NAME.zip" "$Env:APPVEYOR_BUILD_FOLDER\build_$Env:VCMI_BUILD_PLATFORM\cov-int\"
+
+# cf. http://stackoverflow.com/a/25045154/335418
+Remove-item alias:curl
+
+Write-Host "Uploading Coverity analysis result..." -ForegroundColor "Green"
+
+curl --silent --show-error `
+     --output curl-out.txt `
+     --form token="$Env:coverity_token" `
+     --form email="$Env:coverity_email" `
+     --form "file=@$Env:APPVEYOR_BUILD_FOLDER\$Env:APPVEYOR_PROJECT_NAME.zip" `
+     --form version="$Env:APPVEYOR_REPO_COMMIT" `
+     --form description="CI server scheduled build." `
+     https://scan.coverity.com/builds?project=vcmi%2Fvcmi
+
+cat .\curl-out.txt