Workaround Appveyor random failures due to nuget install errors

Add and use nuget-install.cmd based on
https://github.com/appveyor/ci/blob/master/scripts/nuget-restore.cmd

Tools/nuget-install.cmd

@@ -0,0 +1,28 @@
+rem Workaround 'nuget install' not being reliable by retrying a few times
+
+@echo off
+rem initiate the retry number
+set errorCode=1
+set retryNumber=0
+set maxRetries=5
+
+:RESTORE
+nuget install %*
+
+rem problem?
+IF ERRORLEVEL %errorCode% GOTO :RETRY
+
+rem everything is fine!
+GOTO :EXIT
+
+:RETRY
+@echo Oops, nuget restore exited with code %errorCode% - let us try again!
+set /a retryNumber=%retryNumber%+1
+IF %reTryNumber% LSS %maxRetries% (GOTO :RESTORE)
+IF %retryNumber% EQU %maxRetries% (GOTO :ERR)
+
+:ERR
+@echo Sorry, we tried restoring nuget packages for %maxRetries% times and all attempts were unsuccessful!
+EXIT /B 1
+
+:EXIT