github oauth验证现在必须要有user-agent请求头，否则报错

```
Request forbidden by administrative rules. Please make sure your request has a User-Agent header (http://developer.github.com/v3/#user-agent-required). Check https://developer.github.com for other possible causes.
RemoteSocketClosedError: Remote socket was terminated before `response.end()` was called, GET https://api.github.com/user
    at IncomingMessage.<anonymous> (/Users/chemzqm/work/weitalk/node_modules/weibo/node_modules/urllib/lib/urllib.js:346:15)
    at IncomingMessage.EventEmitter.emit (events.js:117:20)
    at _stream_readable.js:920:16
    at process._tickCallback (node.js:415:13)
```
请修复一下