Add support for corporate http(s) proxies, via request proxy option.

[asnowfix]

This is something that is (was) missing from node-jira when trying to use *.atlassian.net from behind a corporate proxy.

[robertlevy]

I'd like this feature as well but it could be implemented much easier by simply tweaking the ctor:

this.request = require('request').defaults({proxy: proxy})

There's no need to set the option on a per-api basis since you're either going to want to send everything through the proxy or nothing through the proxy.

[asnowfix]

@robertlevy nice tip.