Add noproxy configuration

As mentioned in this pull request isaacs/npm/pull/2873, I've added noproxy configuration.
I think registry urls should now respect NO_PROXY if the registry is on that list.

However, tests are failing since *this.conf.get('noproxy')* cannot be retrieved yet.


Associated Pull Request:
isaacs/npm/pull/2873
isaacs/npmconf/pull/13