Add nock to node test helper.

@knolleary,

I think a few node tests (web nodes in particular) will want to use nock which it turns out only works on node >= 0.10. Rather than have lots of tests doing the check against node.version. I thought it could be done here and node tests could just do:
   
    var nock = helper.node;
    ...
    if (nock) {
       it('test requiring nock', ...);
    }


What do you think about doing this? Is there a more elegant way?

-Mark.