Adds `RSpec.current_example`

* Library authors that want to support RSpec 2 and 3
  can use `RSpec.current_example` to find the current example in a hook
  method. In RSpec versions before 2.99, `self.example` can be used (but
  `self.example` is deprecated in RSpec >= 2.99 and removed in RSpec 3).
The example shows how to "memoize" the result of RSpec.respond_to?(:current_example)
Uses thread-local variables

* While RSpec isn't necessarily thread safe right now, it might be in
  the future.
Merge pull request #1011 from alindeman/current_example

RSpec.current_example (2.99)