Relax explicit NUnit and log4net requirements

These were put in place to be compatible with a not-yet-nuget-packaged
verison
of FastDAO. I am now packaging FastDAIO and need to relax these version
requirements.
2.1.2
Update nuget version

All tests pass with the latest version
2.1.3
Make nunit dependency less specific

I am tring to resolve requirement conflicts
2.1.4