add sync popular modules, close #468

- add sync popular modules
    - `config.syncPopular`
    - `config.syncPopularInterval`
    - `topPopular`
- change `ms` to `humanize-ms`
嗯，有道理 +1
2014-10-13 上午11:52于 "Yiyu He" <notifications@github.com>写道：

> In proxy/npm.js:
>
> >      registry: 'http://r.cnpmjs.org', // registry.npmjs.org/-/short is 404 now.
> >    });
> >    return r.data;
> >  };
> > +
> > +exports.getPopular = function* (top, timeout) {
> > +  var r = yield *request('/-/_view/dependedUpon?group_level=1', {
>
> 感觉可以等私有的搞定之后做个私有的，公共的可以直接代理到 NPM ?
>
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> <https://github.com/cnpm/cnpmjs.org/pull/472/files#r18753302>.
>