Support for Mvc5 & FluentAssertions 2.2.0

For Issue https://github.com/CaseyBurns/FluentAssertions.MVC/issues/17
* Updated Nuget, targets to newer, package restore-friendly NuGet approach.
* Changed NUnit dependency to a NuGet dependency
* Updated to FluentAssertions 2.2.0
* Added FluentAssertions.Mvc5 (only targets .Net v4.5)