Better build and push nuget pkgs

This version builds all nuget packages first and then, optionally publishes them.

The reason for this PR is that the current version could not handle pushing nuget packages and symbol packages to two different locations.
xml comments were also missing so this fixes #285 

- 	Update projects to output xml files for Configuration=Release
- 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 FAKE do not expose any helpers
for publishing a specific nuget file so I copied their code.