Feature request: Multi-framework build (.NET 3.5)

These changes satisfy the use cases below for .NET 3.5:

-   Use case 1: I am a ColorMine developer, and I want to, at the click of a button, build assemblies for multiple .NET framework versions from non-duplicated source files.

-   Use case 2: I am a ColorMine developer, and I want to, at the click of a button, build a ColorMine NuGet package that includes all .NET framework builds in my solution.

-   Use case 3: I am a ColorMine developer, and I want to, at the click of a button, build unit tests for multiple .NET framework versions from non-duplicated source files.

-   Use case 4: I am a ColorMine developer, and I want to, at the click of a button, run all unit tests for all included .NET framework builds in my solution.

I will add multi-framework build support for 4.0, 4.5, and 4.5.1 if you accept this pull request.

Please let me know if you have any questions.