Migrate to MbUnit framework

[Alexander Regueiro]

The MsTest (Microsoft's unit testing framework integrated into Visual Studio) has proven itself somewhat lacking in features and even buggy in certain cases. It's effective dependency on Visual Studio is also not desirable, especially for Continuous Integration scenarios.

MbUnit is proposed as a mature and feature-rich test framework to replace the current unit and integration tests for the net40 and sl40 library projects.