Add ability to manually specify the used agent

Hi,

I had a problem with thousands of waiting sockets in TIME_WAIT state. The underlying problem is described here: https://github.com/joyent/node/issues/1958

One possible solution is to use a custom agent like 
https://github.com/TBEDP/agentkeepalive

The patch allows the usage of such an agent with http-get.

Regards
Stefan