Custom delegate types

- Allowing users to to create custom delegate (It, Should, etc.)
delegates using the DelegateUsageAttribute.
- Adding 'leader' support (It => "it", Behaves_like => "behaves like").
- Adding leaders to output in certain places.
Removing leaders from runners

- Removing leaders from runners
- Ensuring that all specs pass
Removing .ToArray() in EnumeratingContextsIn
Adding an example for custom delegates