Refactoring Sandboxing to be in RSpec support

rspec-core changes for https://github.com/rspec/rspec-core/issues/1798

rspec-support changes in https://github.com/rspec/rspec-support/pull/137
So I figured out a pretty simple solution for the `RSpec.current_example` thing:

* Store and restore the original value of `RSpec.current_example` around the sandbox like you are doing for config and world.
* On the new config object, define a `before(:context)` hook that clears it:

``` ruby
new_config.before(:context) { RSpec.current_example = nil }
```