Made the expansion of the test assemblies itemgroup delayed

so it is not executed until after the Build Task. This enables the build
to run on a clean directory, as well as supporting re-runs on directories
with existing test binaries from previous runs.
Specified minimum required xUnit.net version in .nuspec file.
Merge branch 'nuspec-dependency' of https://github.com/moodmosaic/Exude into moodmosaic-nuspec-dependency
Added a first draft of a proper description

to README.md.