Add socks5 proxy

[alchen]

I needed to use socks5 proxy for my Twitter requests, so I made some modifications to the module:

• Added a new request.js to encapsulate request with relevant proxy configs.
• Added a new dependency [mscdex/socksv5](https://github.com/mscdex/socksv5) to use request with socks5. There is only a handful of socks agents and this one seems to be still being maintained and also most up-to-date.
• Moved trusted certificate checking logic into req.on('response', ... because of request 2.65.0 getPeerCertificate() always returns null on node 4.2.1, works on 0.12.7 nodejs/node#3545 (test cases was failing).
• Changed a couple of test cases to accommodate above changes.

The proxy option is enabled by adding something along this line into Twit config: proxy: { host: '127.0.0.1', port: '1080' }

I haven't had a chance to try HTTP proxies. I wonder how well they work with Twitter's https endpoints.

[ttezel]

@alchen could you rebase these changes off latest master branch?

[ttezel]

Could you rebase these changes off latest master so I can re-review?

[alchen]

@ttezel Thanks for the interest of incorporating this. I am however no longer familiar with the changes that I wrote -- I quickly rebased everything and someone else might be able to pick up from here.