Add tests and docs for using the agent, agentClass and agentOptions. Forever option defaults to using http(s).Agent in node 0.12+

[simov]

Fixes #1637
Fixes #1669

Feedback: @mikeal @devtristan @triccardi-systran and anyone else interested in

• Take a look at the agent tests here. Just a bunch of simple use cases, also note that the forever option defaults to http(s).Agent in node 0.12+
• Take a look at the docs as well. A little bit down below there is a section for agent related options. It would be great if you read through all of them carefully and let me know if I'm not getting something right.

[simov]

@mikeal @devtristan @triccardi-systran (+ anyone else) any feedback on this one?

[summer4096]

I don't love forever being 0.10 only, as the docs say, but the code clearly supports the option in more recent versions. Why not get rid of the console.warn and keep forever as an option available for all versions?

[simov]

Probably a more appropriate way to document the forever option would be to say that it defaults to http.Agent in node 0.12+ which was suggested by @mikeal, also that's the way to get rid of potential bugs such as the memory leak one in your (@devtristan) issue here #1637. The console.warn can be removed in that case I agree.

[simov]

@devtristan updated the docs and removed the warning.