Unit Testing Overhaul

I've gone ahead and completely rewritten all of the unit tests using NUnit and fluent assertions. I've also split the unit and integration tests into separate projects, however it might be a better idea to just merge them into one.

With this change I want to get IronGitHub onto a build server with nightly builds to constantly check for compatibility with any api changes, and additionally run tests against new commits and pull requests. You can see an example of this here: http://ci.johnduhart.me/viewType.html?buildTypeId=Test_IronGithubTest&tab=buildTypeStatusDiv