Eliminate infinite recursion when Resque::Helpers mixed into Resque

Some third-party plugins rely on the Resque::Helpers methods being available
in the Resque namespace, as opposed to mixing the helpers module into their
own code. While this is ill-advised, the change in Resque broke expectations
and consumers ended up with weird breakages (see #1150).

This patch enables consumers to mix the Resque::Helpers back into the Resque
module as a workaround until the third-party plugins receive the appropriate
updates:

``` ruby
  # Ill-advised legacy support for gems that hooked into private API.
  # WARNING: This won't work in Resque 2.x
  Resque.extend(Resque::Helpers)
```