Convert project to NetSDK format

[mnivet]

New NetSDK format for csproj, supported since VS2017, provides native NuGet packaging capabilities, which should help to produce a valid PowerShell package later since PowerShell packages are just NuGet packages with conventions specific to PowerShell

This PR is not really adding the support of packaging the dll as a PowerShell package. It's just a preparation step for that.

But this PR is not limited to this, it also fix the reference issue discuss in issue #1 thanks to the comment of martinisoft1, and use more reliable references to System.*.WindowsRuntime with the help of NuGet