Push symbols to server

-Upgraded to latest FAKE 3.4.1
-build.cmd can be started from anywhere
-Let FAKE's NuGetHelper and nuget.exe build symbols packages and let
them decide which files to pick (no pdb and src in normal packages)
by putting everything needed for both packages in workingDir
-packages are pushed in it's own target PublishNuget after all packages
have been created. Unfortunately FAKE's publishing of packages do not allow
different symbol and nuget upload url's and do not expose any helpers
for publishing a specific nuget file so I copied their code.