OSX Support

[andreiborodin]

Adding support for OSX, which should technically work on Linux as well. This fixes the #68 issue.

This change does a couple of things:

• Fixes all path operations with appropriate slash ('/')
• Fixes built-in package search path for macOS.
• Adds Unix .NET Support props file that retargets to Mono
• Fixes up the executable path for dotnet and msbuild.
• Updates Sample's external C# project to work with .NET 4.6 Reference Assemblies.

[mhochk]

    public static BuildTargetGroup GetBuildTargetGroup(BuildTarget buildTarget)

This function appears to be available in Unity starting in 2017.

UnityEditor.BuildPipeline.GetBuildTargetGroup(BuildTarget buildTarget)

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Refers to: Source/MSBuildTools.Unity/Packages/com.microsoft.msbuildforunity/Editor/ProjectGenerator/Scripts/Utilities.cs:576 in 58783b0. [](commit_id = 58783b0, deletion_comment = False)

[mhochk]

    /// Gets a normalized path (with back slashes only), and optionally can make full path.

NIT: Update comment

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Refers to: Source/MSBuildTools.Unity/Packages/com.microsoft.msbuildforunity/Editor/ProjectGenerator/Scripts/Utilities.cs:372 in 58783b0. [](commit_id = 58783b0, deletion_comment = False)

[mhochk]

Looks good.