Upgrade to EF5, .NET 4 + 4.5 versions, plus extras

- Upgrade to EF5 with support for .NET 4 and 4.5
- Ability to use an existing context within a hook (this is from an old pull request)
- Support for stub entities (where you create a stub from just an ID for adding associations in something like an MVC application)
- Fixed unit tests so that they all pass