Interface for injecting custom authorization

I added an interface so that you can inject your own authorization implementation.
We needed this in our project since we were using custom authorization code.

How to use it:
1) Create class implementing the IAuthorizerForElmah interface, which is just the single method IsAuthorizedForElmah(username). In this method you can write your own code for determining whether this user is authorized to use elmah.
2) Inject the class using the Dependency Injection framework you are using in your project. (Elmah.mvc will find the class using the DependencyResolver)