Switch build, pack & push to use `dotnet`

[fluffynuts]

I'd like to be able to hack on PB on any machine - so I'd like to embrace dotnet for build & targeting.

This raises the issue of minimum framework support (currently net452) - though the included MonoForFramework.targets file may suffice with a reasonable system Mono install, conversely the solution at https://stackoverflow.com/a/45380046 would allow build & test on !windows, but release would still have to happen on a windows machine.

This also raises the issue of platform-specific packages:

• PeanutButter.ServiceShell
• PeanutButter.WindowsServiceManagement

These are quite stable and should probably be moved into their own repos, much like https://github.com/fluffynuts/PeanutButter.TrayIcon

If tooling can be switched to dotnet, then packaging metadata would come from the .csproj files, meaning zarro would just build packages (remove custom package building), zarro could increment package versions (remove NugetPackageIncrementer) and the Package.nuspec files could be removed too.

[fluffynuts]

update: minimum framework is now net462, so that's one issue down; ServiceShell and WindowsServiceManagement should move out into their own repo, perhaps PeanutButter.Win32.