Support net framework?

[weianweigan]

Hi, this is a awsome project.
I want to use it in my work, but i found that it cannot used in .net framework's wpf project.
So i forked it and add net461 to TargetFrameworks.

This is my commit.
I changed a few lines of code to compile and passed testcase.
It passed unittests but I don't konw if there is some other problems when i use it in .net framework.
If there is no major problem, Could i create a pr for this?

[mattkol]

@weianweigan sure, please create the PR, it would help other developers using .NET Framework.
Thanks.