Added keep-alive agent to improve performance over https

We were experiencing quite poor response times as node.js doesn't keep the https connections open unless there's a queue of outgoing requests and it therefore had to setup a new https connection on each request. By adding the keep-alive-agent we reduced the response time for a simple request with about 40%.

I couldn't get the tests running (haven't used streamline before..) so I'm not sure if I broke something or not. It's at least working in our environments.