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
I think that rather than this taking a config argument, it's a slightly nicer API (and simpler) to not take an argument, always create a new config instance, and then pass the config instance to the block below.  That way, the caller can do something like (in rspec-core's case):

``` ruby
Sandbox.sandboxed do |config|
  config.expose_dsl_globally = false
  config.expecting_with_rspec = true

  # the rest of the custom sandboxing logic...
end
```

This is slightly less flexible (in the sense that it doesn't allow a caller to maintain a config instance between calls to `sandboxed`), but I think that's a good thing: the purpose of sandboxing is to use a clean-slate config object within the sandbox, so supporting a persistent config object doesn't make sense IMO (and certainly isn't needed in the use cases so far).