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.
For ruby version differences we tend to prefer to define differing versions of methods to make it explicit what environments need which logic and to only incur the overhead at file load time rather than each time the method is called.  In this case, it could be:

``` ruby
if RUBY_VERSION.to_f < 1.9
  if self.instance_variables_for_example(group)
    group.instance_variables - ['@__inspect_output']
  end
else
  if self.instance_variables_for_example(group)
    group.instance_variables - [:@__inspect_output]
  end
end
```