Move OmniSharp to MSBuild for .NET Core projects

[DustinCampbell]

This tracks the initial work to move OmniSharp and the C# extension for VS Code to MSBuild-based .NET Code projects.

• Punch through! Get to IntelliSense displaying in VS Code for an MSBuild-based .NET Core project.
• Update OmniSharp to process a folder path containing .csproj files similarly to how it works for project.json today [PR: https://github.com/Expand MSBuildProjectSystem to load .csproj projects from a root path OmniSharp/omnisharp-roslyn#635]
• Update VS Code extension to properly find launch targets for .csproj files [PR: Update launch targets #760]

[DustinCampbell]

Submitted OmniSharp/omnisharp-roslyn#635 to OmniSharp

[DustinCampbell]

Submitted #760 for launch target discovery.

[DustinCampbell]

v1.5-beta1 released which allows launching of OmniSharp on folders with .csproj files but no .sln file.

[DustinCampbell]

The punch through has happened!

[DustinCampbell]

I'm going to move this over to a GitHub Project for better tracking. Further work will be tracked here