.NET Core-based build system

[ForNeVeR]

I want the whole project (or the most of it) to be built using dotnet tool. Some important thoughts:

1. I want the project to be editable using Visual Studio, so probably the sln file still need to be used. It'll reference csproj and xproj files (or maybe project.json directly?).
2. There's a couple of .NET Core-incompatible projects (Windows Forms controls and samples), I'm not sure whether they need or can be converted (UPD: it seems they can).
3. At the current state project won't support .NET Core itself (see Port to .NET Core #1 for the current state) but I think that dotnet could actually build FullCLR projects; need to check that.

@gsomix and sleepyvenom have shown interest in the task, although I'll obviously have to take a leading role in the implementation.

I've started initial work in the feature/61-dotnet-core branch. This task have a couple of dependencies:

• Test fails on Travis #62: Test fails on Mono 4.6
• Revive examples #46: Revive examples
• Remove Muzzle reference from the main test library #72: Remove Muzzle reference from the main test library
• Convert every project to dotnet (see Convert VbExample to .NET Core #74, currently impossible for VB.NET)
• Rebase [WIP] dotnet-cli build system #63
• Check if the ConsoleClient works
• (new from 2016-12-05) try preview 3 and MSBuild stuff
• Port GUI projects back to MSBuild
• Check if the GUI client works
• Check if the VB.NET example works
• Fix both CI services
• Update the documentation accordingly