Adds `RSpec.current_example`

* Library authors that want to support RSpec < 2.14 and RSpec >= 2.14,
 3.0 can use `RSpec.current_example` to find the current example in a hook
 method. In RSpec versions before 2.14, `self.example` can be used (but
 `self.example` is deprecated in RSpec >= 2.14 and removed in RSpec 3).