Add better `inspect` output for ExampleGroup

Pull Request for #1590 

The only problem I ran into was that the syntax error in the after hook test produces error output while running the spec (the test still passes).

> An error occurred in an `after(:context)` hook.
>   NoMethodError: undefined method `expcts' for #<RSpec::ExampleGroups::SomeClass_3 after(:context) hook>

This is definitely not ideal because it could distract from problems with "real" after hooks. The only two options I could up with were: 1) remove the test, or 2) add a way to suppress that error. I'm interested in your thoughts on the best way to handle this.
Accessing the instance variable directly like this is a code smell, IMO.  I think this spec can be tweaked slightly to retain it's essence without doing that:

``` ruby
      it 'does not clear the inspect output after the example runs' do
        an_example = nil

        line = __LINE__ + 2
        group = ExampleGroup.describe do
          example do
            an_example = self
          end

          before(:context) {}
        end

        group.run

        path = RSpec::Core::Metadata.relative_path(__FILE__)
        expect(an_example.inspect).to end_with("\"example at #{path}:#{line}\">")
      end
```

Thoughts?