Platform specific unit-tests

After splitting the project in 2 libraries targetting at both classic and portable .NET platform it was also crucial to provide unit-tests running on every specific platforms. Therefore it was necessary to split the previous unit test project targeting at .NET 4.0 into 2 other projects also targeting at Windows 8 platform and Windows Phone 8 platform.

Note that for unit testing compatibility accross platforms, NUnit had to be replaced by the MSTest framework. However the test have not been rewritten and are still using FluentAssertions.