Add hook scope aliases `:example` and `:group`

As per [a discussion on the mailing list](http://groups.google.com/group/rspec/browse_thread/thread/26ea96043f46b3b6), Evgeniy Dolzhenko suggested that if we were going to have aliases, it would be optimal to have a change that introduced the aliases of `:example`, and `:group`, to parallel the existing `:suite` alias.

Here is a change that does this.