Issue 58 shouldbeoneof take two

!()[https://raw2.github.com/bendetat/internet-memes/master/rage/frnXxyM.gif]

You've got to be kidding me. Diff issues again. Everything is sane locally but github is showing all of the things in `src/Shouldly.Tests/Shouldly.Tests.csproj` and `src/Shouldly/ShouldBeTestExtensions.cs`. Do we know anybody who works at github? :-P

Anyway I also included ShouldNotBeOneOf. It's hot. Trust me.