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.
Ah, I get your test now.  I think it's a reasonable approach, but we may be able to tweak it a bit to make it more clear:

``` ruby
      it "does not pollute an example's `inspect` output with the inspect ivar from `before(:context)`" do
        inspect_output = nil

        line = __LINE__ + 2
        group = ExampleGroup.describe do
          example do
            inspect_output = inspect
          end

          # define a before(:context) hook so that it initializes the ivar for
          # inspect output, to demonstrate that it does not taint the `inspect`
          # accessible from within the example.
          before(:context) { }
        end

        group.run

        path = RSpec::Core::Metadata.relative_path(__FILE__)
        expect(inspect_output).to end_with("\"example at #{path}:#{line}\">")
      end
```

Thoughts?