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
"Use in place of a Reporter..." sounds like a suggestion for users to use this class.  But we're considering it part of RSpec's private API (which is good), so we don't want users using it directly.  Can we reword this to something like:

``` ruby
# Used in place of a {Reporter} for situations where we don't want reporting output.
```

Thoughts?