Update nunit libs to 2.6.2
Downgrading Nunit from 2.6.2 to 2.6.1. There is an issue with 2.6.2 that prevents it from working on Mono.
Update release notes to include NUnit 2.6.1 update.
Merge pull request #98 from rmboggs/nunit262

Update NUnit to 2.6.1