New Feature: Compatibility with Unity3d 5.4+

I could not get Unity3d 5.4 OSX to work correctly with this project. I did not try Unity3d Windows. It would be a great feature to be able to use a Unity3D package to just install it in a project. I can see that there would be many Unity3d Editor plugins that could be made to take advantage of the libgit2sharp project.

From the latest branch v22:

Unhandled Exception: System.Reflection.ReflectionTypeLoadException: The classes in the module cannot be loaded.
at (wrapper managed-to-native) System.Reflection.Assembly:GetTypes (bool)
at System.Reflection.Assembly.GetTypes () [0x00000] in :0
at Mono.CSharp.RootNamespace.ComputeNamespaces (System.Reflection.Assembly assembly, System.Type extensionType) [0x00000] in :0
at Mono.CSharp.RootNamespace.ComputeNamespace (Mono.CSharp.CompilerContext ctx, System.Type extensionType) [0x00000] in :0
at Mono.CSharp.GlobalRootNamespace.ComputeNamespaces (Mono.CSharp.CompilerContext ctx) [0x00000] in :0
at Mono.CSharp.Driver.LoadReferences () [0x00000] in :0
at Mono.CSharp.Driver.Compile () [0x00000] in :0
at Mono.CSharp.Driver.Main (System.String[] args) [0x00000] in :0

Missing method .ctor in assembly /Users/mike/Projects/TestLibGit2Again/Assets/Plugins/Release/LibGit2Sharp.dll, type System.Runtime.Versioning.TargetFrameworkAttribute
The class System.Runtime.Versioning.TargetFrameworkAttribute could not be loaded, used in LibGit2Sharp
Can't find custom attr constructor image: /Users/mike/Projects/TestLibGit2Again/Assets/Plugins/Release/LibGit2Sharp.dll mtoken: 0x0a00000a

From the branch v17:
I got DLLNotFoundExceptions.

未指定任何仓库文件、测试或入口点。先根据 ReflectionTypeLoadException 和 v17 DLLNotFoundException 重现报告的 Unity3d 5.4+ OSX 故障，然后调查所请求的 Unity package 兼容性；当 package 在所述 Unity 环境中成功安装并加载时，即视为完成。