Reintroduced support for the plain old .NET 4.0. Closes #121
Enabled nuget publishing for the 3.1 branch
Updated the Nuspec file with the correct version and major contributors.
Applied changes of @onovotny to add support for XUnit2 . Fixes #136
Ensured that exceptions that implement IEnumerable<T> are still formatted as exceptions. Fixes #122.
The time span assertions that asserted the difference between to dates and times didn't properly handle UTC. Fixes #127.
Changed ApplyAssertionRulesEquivalencyStep to a more general AggregateEquivalencyStep